The Pizza Planet truck, which made its debut in the original Toy Story will appear in Toy Story 4 as well, though Thomas Jordan told me in won’t appear in a way that viewers might expect.
Several Pixar easter eggs are seen in all, or nearly all, of the studio’s films.
The Luxo Ball, the yellow ball with the red star on it, that was screen in Pixar’s first short Luxo Jr. Can be seen as a sticker on the toy rockers in the image above from the trailer.