Well, assuming that you have a brawl disc that's already set up to start using hacks, and that the filing of your hacks are placed correctly, using a usb is no different than using an SD. The only difference I've noticed is play/loading speed. But if you don't have a lot of hacks it won't be much of a problem.
But, with no brawl disc You'd have to use an emulator or something. I don't know how Dolphin works, but I see a lot of people use it. I don't think there's much you can do w/o a disc. Maybe ask someone with more experience if there's another way to do it.
So I just move everything to my USB? If so can I use a 4gb USB?