He had to threaten legal action to have his name taken off. I think there is only one episode where he would have had no writing credit.

McGruder was involved in the writing of every episode of season four as
much as he was with every previous season. He was fired when he
couldn't write all 20 of the episodes he contracted to write within a reasonable timetable and then
was absent throughout post production.

It was actually changed because Chris Brown had popped up in the news a couple weeks previously and AS felt that made the show seem "current" after being gone for a long time.

They ordered 20 episodes only ten were delivered. Other than not airing the good times episode first as it obviously sets up the whole season, Adult swim isn't really to blame for the oddness of the season. The writing staff (McGruder) was unable to write all the episodes in the agreed upon amount of time so they…

An even half of the scripts this season are solely about grandad (stinkmeaners episode being counted in them). I'm assuming that they were going to have some other things going on before they cut the episode order in half.