Unless you really need it NOW, I would wait. I'm NOT a fan of the SG3 -- as I've had more than my fair share of problems with it.
The SG4 was going to be announced at CES, THEN it was going to be announced at the World Mobility Conference, THEN Samsung said they would announce it on their own, presumably, later.
In the meantime, HTC and Motorola are bringing out new phones all the time.
I would take a hard look at them before jumping into a Samsung phone.