(spoiler)

One more vote for watching seasons 1 and 2! Season 3 is a mess, so you could potentially stop watching at the season 2 finale and pretend that's the end (ends right before their first book is published).

Season 2 is great; you could probably stop watching there since it comes to a nice stopping point in their careers (right before their first book is published). Season 3 is a mess but season 4 had a bunch of good episodes with a kind of rushed wrap up.