Pixar movie sequels have me mixed. I don't think there was certainly any bad one, and most of the ones they've done so far were ones Disney threatened to do without them if they left. And better yet, the Pixar sequels were far better than the DTV stuff Disney proposed. Monsters University may not have been better than the original, but was used to expand their universe, flesh out the characters more, and to not hurt the heartwarming ending of the original film. The Disney Circle 7 (or 9 or whatever) movie was basically Monsters Inc, but with the roles reversed, and them running around a panicking human world. Toy Story 3 would have been about Buzz being recalled and the unfunny broken toys he'd meet on the way. And I'd assume Finding Nemo 2 would have been "AHHH! Nemo somehow got lost again" instead of expanding Dory's character.
And let us not forget one thing. Pixar has the lower original movies to sequel ratio of most CGI cartoon studios. Sure, their first sequel (Toy Story 2) followed the original with one movie (Bug's Life) in between. And that was pressure from Disney to make a cheap, throwaway DTV title. Instead they treated it like a movie instead of a cash grab, and we got something special. The only other time a sequel followed the other film that closely was Cars 2, but that was a kid's movie anyway. Pixar's allowed to not make a movie that's an emotional roller coaster once in a while. Though Cars 3 has a pliotline to be proud of, unlike the Saturday morning animated series tie in story they used.
Toy Story 4 has me... conflicted isn't even the word. I'd agree it would probably work better as a TV special, and I really think Pixar should be doing more of those and with more characters besides Toy Story. I'm willing to have an open mind about it, even though the movies closed nicely with 3 and the supplemental specials and shorts are more than good enough for the characters.
That said, I think Pixar owes us a Monsters Inc special. Halloween's too on the nose, though.