The original announcement of the film stated that it would open up a new chapter in the lives of Woody, Buzz, and the rest of the gang. Now we've had some confirmation on exactly what that chapter will be about, courtesy of John Lasseter at D23:
"Toy Story 4 is a love story. It's a love story between Woody and Bo Peep. Bo's returned for Toy Story 4. Woody and Buzz head out to find Bo."
He also stated that Bo Peep reminds him a lot of his wife, and that this is a particularly person story for him. There were hints of a romantic relationship between Woody and Bo in the previous films, although it barely even qualified as being a plot point. In Toy Story 3 she was missing, one of the toys given away between 2 and 3, and Woody looks sad when her name is mentioned. Clearly the writers feel that there's enough of a story to be explored there, and it will be something different, mixing the classic adventure/rescue elements of Toy Story with a romantic comedy. Given Up managed to tell a better love story in its opening 10 minutes than most romantic films tell in 120, there is cause for optimism that Pixar can pull this off.