1. correct; also you can say "не могу понять" = "can't understand", "не понимаю" = "don't understand"
2. correct, if a person just said a sentence and asking is it clear; if someone is explaining something to you and action isn't finished, you can say "Вы понимаете?" "Нет, не понимаю".
3. correct, here it is definitely past tense, because "that I said" at the end.
4. correct

Oh, sh.it there is the tense in the words But I think my information will be useful, so I didn't delete it.