Hmm... are you sure your timing is good? If you can do the three-ball flash, you're essentially throwing one ball, then doing one under-the-ball throw on each side. In order to do the fourth throw, all you have to do is toss in another under-the-ball throw identical to the one you did for the second ball. If your timing is solid, you shouldn't have too much trouble tossing in that extra fourth one.
Feel free to tape yourself doing it, maybe I could help you out more if I saw what you were doing. Good luck! :)