Ex 3:
1. after
2. to
3. about
4. on
ex 4:
1. This book is being discussed at the moment (или now).
2. When I entered (или came in), he was being laughed at by everyone (или просто everyone was laughing at him).
3. When we came, the dress had been sold.
4. The house will have been sold by the end of the month.