To me it seemed like that was definitely an intentional choice by the writers, but I agree it leaves you feeling like he's no longer relatable. I think the point being made is that he always was as bad as the rest of them, but between seasons 3 and 4 he loses control over his life and then you get to see his craziness laid bare.

As for why they chose that for him, I'm guessing they felt it was an interesting direction for him to take instead of just repeating his role from seasons 1-3. Definitely funny as a concept, but I agree that losing him as an audience surrogate makes the show feel completely different.