Answer:
Kinda. The difference is that a supposition has some facts to go on, a guess is pure random. A supposition is kinda like playing pretend, more than guessing.
For instance, "suppose" that you are a frog. What would you do? See, we have some ideas about what frogs do, so we have some facts to go on. A guess is just random like "what is life like on the Moon?" We don't have any facts, so the story would be a guess.