How to sample nearby items while keeping performance?

Each of my streams is ordered, and I'd like to be able to sample N (4 ish) nearby items. Is there an idiomatic way to do this in Mosaic while keeping shard pre-download etc?

Concretely, what I want is to be able to express:

stream ~ p(streams)
s_i, s_i+1, s_i+2, s_i+3 ~ p(stream) 