According to the strict rules, if you have two complete sentences joined by the "and" you use the comma. So, your should NOT have the comma, the second should, the third should not, and the fourth should. In practice, if the two independent clauses are very short, that comma is often omitted, and if you want the speaker to pause -- to draw attention to the contrast, for example -- you can put in the comma even if you don't repeat the subject.
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