Toy Story 4 is coming out this year and though many saw it as an unnecessary sequel, it does have one plot point that fans are curious about; Bo Peep's whereabouts. Toy Story 3 made it clear that she and a number of Andy's older toys were sold one day but some new footage from CinemaCon actually gives us better insight on that fateful day.
According to IGN SEA, the film has a prologue that takes place nine years ago when Andy's family decided to sell a bunch of his toys. It's a proverbially dark and stormy night when the toys stage a rescue mission to save RC. The toy car has been left outside in the rain and is about to get washed away when its fellow toys come to its aid. Forming a human, er, toy chain, Woody is lowered down from a window in Molly's room and rescues RC just in time. But they're not out of danger yet.
Sadly, while they are able to save RC, Bo Peep and a number of other toys were put into a box to be sold away. Woody tries to save them but Bo Peep tells him not to, saying this is just life for toys and that she's one of Molly's, accepting her fate. The two have a sweet goodbye before Andy takes Woody back up.
It will be interesting to see how they reunite, especially now that Bo Peep has a new costume and a (presumably) more headstrong attitude. My guess is, they meet in that amusement park.
Toy Story 4 comes out on June 21.