They look fine to me. I might suggest for 4. It has been (or It's been) a long time since I met him.
I generally sense that the "have" in the original "have met him" should be reserved for things that had a duration in time, when they happened. "It's been a long time since I have eaten with him" - because eating takes time, whereas when you met him was an exact point in time.
Others may see it differently?
Student or Learner