I have been consistently surprised by how good each of the Toy Story films was. I can't think of a way to make 5 after the way 4 ended (unless it has new protagonists) but I expect the folks making the movie to surprise me again.
They were all good films. 4 had a very definitive "end of story, close the book" feel to it. It wrapped everything up nicely. It's going to be very hard not to ruin it but we'll see.