The Toy Story Trilogy is one of the rare perfect trifectas in film history. An original film that was a trail-blazer in animation, followed up by an unlikely sequel that began as a direct-to-video cash-in then wound up even better than the original, and capped a heartbreaking/warming finale that remains a high water mark for Pixar Animation's signature mixture of whimsy and quietly devastating emotion.
There was absolutely no need for a fourth film.
So, of course, Pixar went and made one anyway, and it's brilliant.