We know that Bo Peep had a very close relationship with Woody, so it’s not surprising that her return would be important to him, but she’s clearly been leading a very different life than he has these past several years, and that will seemingly influence Woody in some way.
Bo Peep disappeared after Toy Story 2, and was only briefly referenced in the third film.
It’s not made clear where she went or why, though those details have been revealed through footage in the new movie that was shown at CinemaCon.