1.) correct ( from time to time ) 2.) / 3.) I haven't been seeing you.................... (continuous basis) 4. We've seen him......................

All are grammatically correct. 1-3 are close in meaning but not precisely the same. In #3, "since" will be interpreted to mean "because". In #4, we have seen him one or more times since he left. #2 can be a useful sentence: "I have been busy. I still have time to study English and take care of the children. Unfortunately, I have not been seeing you since war broke out." In this sentence, "since" can mean "since the time" or "because". The precise meaning can only be determined with additional context. #3: This is a very good sentence since "since" can have the same meaning as "because". Thus, I would interpret #3 as "I have not been seeing you because war has broken out." It is more difficult, but not impossible, to make sense of #3 if you interpret "since" to mean "since the time". It would not be an error, just a bit awkward. If you want the meaning "since the time", #2 is better.

1 and 4 are correct. The verb -see- is a stative verb , describing state of ' being 'rather than action, though we can sometimes using the verb -see-as a dynamic or active verb eg do you have a girlfriend? Yes, I am seeing someone at the moment (dating). In your example -see- can not be used in the continuous form. sentence 4 doesn't give us information about how many times, just that we have had the experience of seeing him. Hope that helps!