I didn't like Toy Story 3, it's alright but many of the things people love it for where already done in previous movies like The Brave Little Toaster. Toy Story 2 is also alright. Don't get me wrong they're both fine movies but the only stand out film is the original. I would've preferred the original concept for Toy Story 3 which was about Buzz being recalled:
People acted like it would've been terrible because it was a Disney film instead of Pixar but in hindsight nearly ten years later we can now see that Disney can make some great movies on their own and Pixar can make some great stinkers by themselves as well. I mean even when TS3 was announced by Pixar people were whining about how now that Disney owned Pixar they were forcing them to make a third film.
I do think these new movies will have a subtitle and won't be "TS4" etc. They probably want to brand to continue for many years so they'll stop numbering them and people who grew up with the originals will be ok that "their" trilogy is still intact. I'm sure all upcoming TS movies will be fine since they know it's Pixar's most iconic IP. I'd be more concerned with the other sequels like Finding Dori and the Incredibles.