diff --git a/app/src/main/AndroidManifest.xml b/app/src/main/AndroidManifest.xml
index 9312351..5720e3c 100644
--- a/app/src/main/AndroidManifest.xml
+++ b/app/src/main/AndroidManifest.xml
@@ -3,6 +3,7 @@
     package="com.magdamiu.androidfundamentalsmai2021">
 
     <uses-permission android:name="android.permission.INTERNET" />
+    <uses-permission android:name="android.permission.CALL_PHONE" />
 
     <application
         android:allowBackup="true"
@@ -11,7 +12,8 @@
         android:roundIcon="@mipmap/ic_launcher_round"
         android:supportsRtl="true"
         android:theme="@style/Theme.AndroidFundamentalsMai2021">
-        <activity android:name=".activities.SecondActivity"></activity>
+        <activity android:name=".code_challenge_4.ImplicitActivity" />
+        <activity android:name=".activities.SecondActivity" />
         <activity android:name=".activities.FirstActivity" />
         <activity android:name=".ConstraintLayoutSampleActivity" />
         <activity android:name=".MainActivity" />
diff --git a/app/src/main/java/com/magdamiu/androidfundamentalsmai2021/StartActivity.java b/app/src/main/java/com/magdamiu/androidfundamentalsmai2021/StartActivity.java
index 3a1a9c0..7707700 100644
--- a/app/src/main/java/com/magdamiu/androidfundamentalsmai2021/StartActivity.java
+++ b/app/src/main/java/com/magdamiu/androidfundamentalsmai2021/StartActivity.java
@@ -7,6 +7,7 @@
 import android.view.View;
 
 import com.magdamiu.androidfundamentalsmai2021.activities.FirstActivity;
+import com.magdamiu.androidfundamentalsmai2021.code_challenge_4.ImplicitActivity;
 
 public class StartActivity extends AppCompatActivity {
 
@@ -48,4 +49,9 @@ public void startFirstActivityOnClick(View view) {
         Intent intentToStartFirstActivity = new Intent(StartActivity.this, FirstActivity.class);
         startActivity(intentToStartFirstActivity);
     }
+
+    public void startImplicitActivityOnClick(View view) {
+        Intent intentToStartImplicitActivity = new Intent(StartActivity.this, ImplicitActivity.class);
+        startActivity(intentToStartImplicitActivity);
+    }
 }
\ No newline at end of file
diff --git a/app/src/main/java/com/magdamiu/androidfundamentalsmai2021/code_challenge_4/ImplicitActivity.java b/app/src/main/java/com/magdamiu/androidfundamentalsmai2021/code_challenge_4/ImplicitActivity.java
new file mode 100644
index 0000000..8a67f56
--- /dev/null
+++ b/app/src/main/java/com/magdamiu/androidfundamentalsmai2021/code_challenge_4/ImplicitActivity.java
@@ -0,0 +1,92 @@
+package com.magdamiu.androidfundamentalsmai2021.code_challenge_4;
+
+import androidx.appcompat.app.AppCompatActivity;
+
+import android.annotation.SuppressLint;
+import android.content.Intent;
+import android.net.Uri;
+import android.os.Bundle;
+import android.view.View;
+import android.widget.EditText;
+import android.widget.Toast;
+
+import com.magdamiu.androidfundamentalsmai2021.R;
+import com.magdamiu.androidfundamentalsmai2021.activities.FirstActivity;
+
+import static com.magdamiu.androidfundamentalsmai2021.R.string.use_device_with_sim;
+
+public class ImplicitActivity extends AppCompatActivity {
+
+    private EditText editTextWebsite, editTextLocation, editTextShareText, editTextPhoneNumber;
+
+    @Override
+    protected void onCreate(Bundle savedInstanceState) {
+        super.onCreate(savedInstanceState);
+        setContentView(R.layout.activity_implicit);
+        findViews();
+    }
+
+    private void findViews() {
+        editTextWebsite = findViewById(R.id.editTextWebsite);
+        editTextLocation = findViewById(R.id.editTextLocation);
+        editTextShareText = findViewById(R.id.editTextShareText);
+        editTextPhoneNumber = findViewById(R.id.editTextPhoneNumber);
+    }
+
+    public void openWebsiteOnClick(View view) {
+        Intent openWebsite = new Intent(Intent.ACTION_VIEW);
+        String urlOfWebsite = editTextWebsite.getText().toString();
+        if (urlOfWebsite.isEmpty()) {
+            editTextWebsite.setError(getString(R.string.error_insert_url));
+        } else {
+            openWebsite.setData(Uri.parse(urlOfWebsite));
+            startActivity(openWebsite);
+        }
+    }
+
+    public void openLocationOnClick(View view) {
+        String location = editTextLocation.getText().toString();
+        if (location.isEmpty()) {
+            editTextLocation.setError(getString(R.string.error_insert_location));
+        } else {
+            Uri gmmIntentUri = Uri.parse("geo:37.7749,-122.4192?q=" + Uri.encode(location));
+            Intent openLocation = new Intent(Intent.ACTION_VIEW, gmmIntentUri);
+            openLocation.setPackage("com.google.android.apps.maps");
+            startActivity(openLocation);
+        }
+    }
+
+    @SuppressLint("QueryPermissionsNeeded")
+    public void shareTextOnClick(View view) {
+        String textToShare = editTextShareText.getText().toString();
+        if (textToShare.isEmpty()) {
+            editTextShareText.setError(getString(R.string.error_insert_text));
+        } else {
+            Intent shareText = new Intent(Intent.ACTION_SEND);
+            shareText.setType("text/plain");
+            shareText.putExtra(Intent.EXTRA_SUBJECT, getString(R.string.share_subject));
+            shareText.putExtra(android.content.Intent.EXTRA_TEXT, textToShare);
+            if (shareText.resolveActivity(ImplicitActivity.this.getPackageManager()) != null) {
+                startActivity(Intent.createChooser(shareText, getString(R.string.share_using)));
+            } else {
+                Toast.makeText(ImplicitActivity.this, "No app found on your phone which can perform this action", Toast.LENGTH_SHORT).show();
+            }
+        }
+    }
+
+    @SuppressLint("QueryPermissionsNeeded")
+    public void callPhoneNumberOnClick(View view) {
+        String phoneNumber = editTextPhoneNumber.getText().toString();
+        if (phoneNumber.length() != 10) {
+            editTextPhoneNumber.setError(getString(R.string.error_insert_phone_number));
+        } else {
+            Intent callPhoneNumber = new Intent(Intent.ACTION_DIAL);
+            callPhoneNumber.setData(Uri.fromParts("tel", phoneNumber.trim(), null));
+            if (callPhoneNumber.resolveActivity(ImplicitActivity.this.getPackageManager()) != null) {
+                startActivity(callPhoneNumber);
+            } else {
+                Toast.makeText(ImplicitActivity.this, use_device_with_sim, Toast.LENGTH_LONG).show();
+            }
+        }
+    }
+}
\ No newline at end of file
diff --git a/app/src/main/res/layout/activity_implicit.xml b/app/src/main/res/layout/activity_implicit.xml
new file mode 100644
index 0000000..e38dc87
--- /dev/null
+++ b/app/src/main/res/layout/activity_implicit.xml
@@ -0,0 +1,105 @@
+<?xml version="1.0" encoding="utf-8"?>
+<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:app="http://schemas.android.com/apk/res-auto"
+    xmlns:tools="http://schemas.android.com/tools"
+    android:layout_width="match_parent"
+    android:layout_height="match_parent"
+    tools:context=".code_challenge_4.ImplicitActivity">
+
+    <EditText
+        android:id="@+id/editTextWebsite"
+        android:layout_width="0dp"
+        android:layout_height="wrap_content"
+        android:layout_marginStart="24dp"
+        android:layout_marginTop="@dimen/medium_space"
+        android:layout_marginEnd="@dimen/medium_space"
+        android:autofillHints="@string/website"
+        android:ems="10"
+        android:hint="@string/website"
+        android:inputType="textWebEditText"
+        app:layout_constraintEnd_toEndOf="parent"
+        app:layout_constraintStart_toStartOf="parent"
+        app:layout_constraintTop_toTopOf="parent" />
+
+    <Button
+        android:id="@+id/buttonWebsite"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_marginTop="@dimen/space_8"
+        android:onClick="openWebsiteOnClick"
+        android:text="@string/open_website"
+        app:layout_constraintStart_toStartOf="@+id/editTextWebsite"
+        app:layout_constraintTop_toBottomOf="@+id/editTextWebsite" />
+
+    <EditText
+        android:id="@+id/editTextLocation"
+        android:layout_width="0dp"
+        android:layout_height="wrap_content"
+        android:layout_marginTop="@dimen/medium_space"
+        android:layout_marginEnd="@dimen/medium_space"
+        android:autofillHints="@string/location"
+        android:ems="10"
+        android:hint="@string/location"
+        android:inputType="text"
+        app:layout_constraintEnd_toEndOf="parent"
+        app:layout_constraintStart_toStartOf="@+id/editTextWebsite"
+        app:layout_constraintTop_toBottomOf="@+id/buttonWebsite" />
+
+    <Button
+        android:id="@+id/buttonLocation"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_marginTop="@dimen/space_8"
+        android:onClick="openLocationOnClick"
+        android:text="@string/open_location"
+        app:layout_constraintStart_toStartOf="@+id/editTextLocation"
+        app:layout_constraintTop_toBottomOf="@+id/editTextLocation" />
+
+    <EditText
+        android:id="@+id/editTextShareText"
+        android:layout_width="0dp"
+        android:layout_height="wrap_content"
+        android:layout_marginTop="@dimen/medium_space"
+        android:layout_marginEnd="@dimen/medium_space"
+        android:autofillHints="@string/text"
+        android:ems="10"
+        android:hint="@string/text"
+        android:inputType="text"
+        app:layout_constraintEnd_toEndOf="parent"
+        app:layout_constraintStart_toStartOf="@+id/editTextWebsite"
+        app:layout_constraintTop_toBottomOf="@+id/buttonLocation" />
+
+    <Button
+        android:id="@+id/buttonShareText"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_marginTop="@dimen/space_8"
+        android:onClick="shareTextOnClick"
+        android:text="@string/share_text"
+        app:layout_constraintStart_toStartOf="@+id/editTextShareText"
+        app:layout_constraintTop_toBottomOf="@+id/editTextShareText" />
+
+    <EditText
+        android:id="@+id/editTextPhoneNumber"
+        android:layout_width="0dp"
+        android:layout_height="wrap_content"
+        android:layout_marginTop="@dimen/medium_space"
+        android:layout_marginEnd="@dimen/medium_space"
+        android:autofillHints="@string/phone_number"
+        android:ems="10"
+        android:hint="@string/phone_number"
+        android:inputType="phone"
+        app:layout_constraintEnd_toEndOf="parent"
+        app:layout_constraintStart_toStartOf="@+id/editTextWebsite"
+        app:layout_constraintTop_toBottomOf="@+id/buttonShareText" />
+
+    <Button
+        android:id="@+id/buttonPhone"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_marginTop="@dimen/space_8"
+        android:onClick="callPhoneNumberOnClick"
+        android:text="@string/call"
+        app:layout_constraintStart_toStartOf="@+id/editTextPhoneNumber"
+        app:layout_constraintTop_toBottomOf="@+id/editTextPhoneNumber" />
+</androidx.constraintlayout.widget.ConstraintLayout>
\ No newline at end of file
diff --git a/app/src/main/res/layout/activity_start.xml b/app/src/main/res/layout/activity_start.xml
index 4d971ee..3421317 100644
--- a/app/src/main/res/layout/activity_start.xml
+++ b/app/src/main/res/layout/activity_start.xml
@@ -11,24 +11,30 @@
     <Button
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
-        android:text="@string/start_mainactivity"
-        android:onClick="startMainActivityOnClick"/>
+        android:onClick="startMainActivityOnClick"
+        android:text="@string/start_mainactivity" />
 
     <Button
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
-        android:text="@string/start_recyclerview_activity"
-        android:onClick="startRecyclerViewActivityOnClick"/>
+        android:onClick="startRecyclerViewActivityOnClick"
+        android:text="@string/start_recyclerview_activity" />
 
     <Button
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
-        android:text="@string/start_constraint_activity"
-        android:onClick="startConstraintActivityOnClick"/>
+        android:onClick="startConstraintActivityOnClick"
+        android:text="@string/start_constraint_activity" />
 
     <Button
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
-        android:text="@string/start_firstactivity"
-        android:onClick="startFirstActivityOnClick"/>
+        android:onClick="startFirstActivityOnClick"
+        android:text="@string/start_firstactivity" />
+
+    <Button
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:onClick="startImplicitActivityOnClick"
+        android:text="@string/start_code_challenge_4" />
 </LinearLayout>
\ No newline at end of file
diff --git a/app/src/main/res/values/dimens.xml b/app/src/main/res/values/dimens.xml
index e8b2c28..dccbcc9 100644
--- a/app/src/main/res/values/dimens.xml
+++ b/app/src/main/res/values/dimens.xml
@@ -2,6 +2,7 @@
 <resources>
     <dimen name="medium_text">23sp</dimen>
     <dimen name="medium_space">16dp</dimen>
+    <dimen name="space_8">8dp</dimen>
     <dimen name="space_128">128dp</dimen>
     <dimen name="space_64">64dp</dimen>
 </resources>
\ No newline at end of file
diff --git a/app/src/main/res/values/strings.xml b/app/src/main/res/values/strings.xml
index d4dcf4b..82cbfae 100644
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -12,6 +12,7 @@
     <string name="please_add_your_name_here">Please add your name here</string>
     <string name="greetings">Greetings :)</string>
     <string name="error_insert_name">Please insert a name</string>
+    <string name="error_insert_url">Please insert an url</string>
     <string name="textview_3">TextView 3</string>
     <string name="textview_2">TextView 2</string>
     <string name="textview_1">TextView 1</string>
@@ -66,4 +67,17 @@ Ut fermentum feugiat lorem sed ultrices. Phasellus et mattis diam. Sed faucibus
     <string name="hello_from_first_activity">hello from first activity</string>
     <string name="sample_startactivityforresult">Sample startActivityForResult</string>
     <string name="confirm_message">Yes, I\'m here! :)</string>
+    <string name="start_code_challenge_4">Start Code Challenge 4</string>
+    <string name="website">Website</string>
+    <string name="location">Location</string>
+    <string name="open_website">Open Website</string>
+    <string name="open_location">Open Location</string>
+    <string name="text">Text</string>
+    <string name="share_text">Share Text</string>
+    <string name="phone_number">Phone Number</string>
+    <string name="error_insert_location">Please insert a location</string>
+    <string name="error_insert_text">Please insert a text to share</string>
+    <string name="share_subject">Sharing a text</string>
+    <string name="share_using">Share using</string>
+    <string name="error_insert_phone_number">Please insert a valid phone number</string>
 </resources>
\ No newline at end of file
diff --git a/app/src/main/res/values/themes.xml b/app/src/main/res/values/themes.xml
index 2868c13..df069fd 100644
--- a/app/src/main/res/values/themes.xml
+++ b/app/src/main/res/values/themes.xml
@@ -1,6 +1,6 @@
 <resources xmlns:tools="http://schemas.android.com/tools">
     <!-- Base application theme. -->
-    <style name="Theme.AndroidFundamentalsMai2021" parent="Theme.MaterialComponents.DayNight.DarkActionBar">
+    <style name="Theme.AndroidFundamentalsMai2021" parent="Theme.AppCompat.Light">
         <!-- Primary brand color. -->
         <item name="colorPrimary">@color/purple_500</item>
         <item name="colorPrimaryVariant">@color/purple_700</item>