1. While I was walking on the sidewalk, I was playing a game on my smartphone.

Question

2. When I was walking on the sidewalk, I was playing a game on my smartphone.
(Is #2 grammatical?)

3. While I walked on the sidewalk, I played a game on my smartphone.

4. When I walked on the sidewalk, I played a game on my smartphone.
(Can we use both #3 and #4?)

Answer 1

Yes, they are all correct. The verb tenses match in both clauses in all four.