1. Imagine you have a new pen pal from America. Then ask about 4 questions of him.

2. Pretend you have a new pen pal from America. Then ask about 4 questions of him.

3. Suppose you have a new pen pal from America. Then ask about 4 questions of him.

4. Think you have a new pen pal from America. Then ask about 4 questions of him.

(Are all the expressions grammatiacl? Do we have to use 'had' instead of 'have'?)

2 answers

grammatical? (mistake).....
All are fine except #4. The verb 'think' doesn't work well in this context.

Also, I'd delete 'about' in your sentences. It's not incorrect. It just reads awkwardly.