Rocky 4 is certainly the best of the sequels after the original.
Definitely still do Rocky 5 ( . . . THOUSAND hahaha (spaceballs)). I won't lie in saying that yes it is the weakest of the series . . . but that doesn't make it a bad film. Still has a good touching/emotional story and message that is relatable and always (what I refer to as) a good Sunday film to battle the Sunday Scaries with.
Also you may want to consider Sly's film he did for Cannon Films after Rocky 4 . . . OVER THE TOP 87'. Good father/son story and I will actually say, was better than Rocky 5 even.