Actually, there has been a lot of confusion in this area - a Toy Story 4 was never announced. There is evidence to the fact that Pixar is developing a half-hour television special starring the characters, and that there are more Toy Story shorts on the way. However, there is no evidence that another Toy Story feature film is being made.
I agree with you - Toy Story 3 had a perfect ending. Continuing the story after that in another feature film is a big risk, as it could potentially ruin the emotional core of the third film. I think Pixar realizes that, which is why we are getting more shorts and a TV special that utilizes these characters without stepping on Toy Story 3’s toes.
After all, that is why we are getting Monsters University, a prequel (not a sequel) to Monsters, Inc. - the original film ended beautifully and even though we would love to see what happened to Boo on screen, it would wreak havoc upon the perfect ending we have created for her in our minds.