We don't really know what happened behind the scenes. It did seem like Justin wanted out of the show after the third season and the reason they said he's dead is because it seemed like he wasn't coming back. Then he did and they had to go back on their word because why wouldn't they want him back?

I would totally disagree about season 4. That season of the show barely had any laughs and was all drama. It put Frank's antics to the side and introduced pointless character Sammy, who for some reason, is still on the show. Season 5 so far, is a return to form.