BF4 is speculation really, too far off. Having said that, it does use an advanced version of the BF3 engine, so it's not a bad place to start.
A HD7750 is a good entry level card for right now, but how good it will be in a year or so is pure speculation. I think it's safe to say that it 100% won't play BF4 on high, and that if you want to have a solid gaming card in a year time you either buy well (for your display resolution) or you wait.