I went through the owner'* manual, as well as the GM servicing disk, and those show the cooling system capacity as 10 quarts. As you have managed to get in 4, you're only at a 40/60 instead of the recommended 50/50.
As it is, you'd have less corrosion protection, as well as less boil-over and freezing protection. If the system is full, I'd suggest draining maybe 3/4th or so out of the radiator, and then refilling with 100% antifreeze. That should get you around the recommended amount.