Britta caught that Troy and became basically as dumb but none of the rare moments of insight.
Basically writers were kinda done with her being a halfass hippie rage against the machine contrarian so when she made her major psychology it was an excuse for her to pseudo-analyze any character so they could have breakthroughs and get in touch with their feelings.
Jeff even makes a joke later on by s5 I think where he says when I first met you I thought you were the mature and smart one. They laid off by s6 though.
The show belabors this by always getting meta about the inevitable end and the need to oppose change but they basically provided answers for why the show couldn't go on the way characters changed or didn't chang.
After binging myself I really cant say s4 is the worst. I rewatched with none of the info of Harmon being fired and coming back later present in mind and I enjoyed that season up until the finale. I never noticed a dip in quality. To me its more noticeable with s1 getting better as they find their footing and the show trying to stay the same despite Pierce, Troy, and then Shirley leaving.
Chang rose up my fav character list. Also the background characters. They all stayed for the 6 seasons just for the odd side roles and call back jokes. Fat Neil, Leonard, Todd, Garrett (lowkey MIP and MVP of recurring characters), that pottery class doctor, Stacey, etc. Playing ppl that spent 6+ years in community college
Also one thing that bothered me about Paget Brewster is that the very season before she played somebody in charge of the computer dept in the episode where Annie and Hickey basically play the game to bribe the entire faculty just to get a bulletin board fixed. She plays the janitors/custodians being able to see any kind of porn in exchange for something. She plays that character once. Then she comes back as new character Frankie and nothing is mentioned. Not even later in the season where they actually need the character she played and its just excused away that she's a not in. No jokes even attempted.