I've been thinking about this recently but I really hope Toy Story 4 has a nice original song!
Toy Story 1 and 2 had a few but the ones that stick out are "You've Got a Friend in Me" from TS1 and "When She Loved me". Other Pixar films (namely Monsters Inc's "If I Didn't Have you") have had similar touching songs (Coco had some beautiful songs, but in a different way that wouldn't work in Toy Story and vice versa) but Toy Story 3 only gave us... one forgettable song which I had to Google, and that was outshadowed by the Spanish cover of "You've Got a Friend in Me" </3
Basically I'm hoping we get a song similar and emotional to some of the songs in Pixar films in the earlier days. I wanna walk out the cinema wanting to go home to listen to whatever song they wrote for the film