A 4 X 8 piece of plywood will never be flat enough
Plywood is just not flat enough to build a a table on. It's also 2 ft too short. It's also too flimsy. Why not listen to someone who has actually built what you are intending to make? If your top ends up with a twist, you'll never get rid of it.
4 ft levels are cheap enough you could buy two of them and not have to switch back and forth to check you supports. Just make sure they tread the same when you buy them. You will always need them for projects later on. I probably have 3 of them, about 5 - 2fts and a 6 ft one.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)