Brief Answer: The first one
The first one is two main clauses which hang together by the frail power of a comma.
The second one is a subordinate clause followed by the main clause. The comma is correct.
The third one is the same kind of sentence as the second one, and the comma is just as correct.
The commas in the last one either separate members of a list or separate the main clause (which is the first part of the sentence), from the other clause (which also is a main clause). Why isn't this sentence incorrect? It isn't because the two clauses are really also separated by but which is a coordinate conjunction. A coordinate conjunction separates two main completely correct clauses.