Well, first you need to have a good understanding of the Minecraft game mechanics and the story elements of the Story Mode. Then, you can start by collecting the necessary resources and following the specific instructions provided in the Story Mode.
Well, it's not mandatory. But watching Toy Story 3 first might give you some background and enhance your understanding of the characters and their development. However, you'll still be able to follow and enjoy Toy Story 4 on its own.