A comedy’s third and fourth seasons are akin to litmus tests of how a show will fair in the future. For example, “Seinfeld” didn’t lose a step in seasons 3 and 4, and from there it stayed consistently funny until…well, until its last episode. On the other hand, “Scrubs” began to show its cracks by the end of its third season, and was in complete free fall by season 5. The same goes for “Weeds.”