We had the pleasure of seeing Toy Story 4 this past weekend and it was such a treat!
The new addition of "Forky" was my favorite part, by far. Tony Hale just cracks me up!
No, you don't have to have seen the others on order to understand what's going on - they do a great summary at the beginning of the previous three films. There's a lot going on, so seeing it in a theater is probably best. Enjoy!