I dont believe you ever told us what size subs you ordered. If you did, I missed it. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif
If they are 12's, 4 cubes net means more like 1.5 cubes each after port and driver displacement. Im not familiar with the G5, but for most 12's that would be on the small side, which would hurt your low-end output. If you go sealed, or they are 10's, you should have plenty of room though.
Do not skimp on the enclosure design in order to use larger subs, or go ported. In my experience, trying to squeeze two 15's into too small of an enclosure in too small of a trunk (for example) many times performs worse than running two 12's in the optimum box. Not saying you are necessarily doing that here, just warning to keep it in mind.