I'd say it could happen, but not in anything resembling a consumer machine--if the power consumption and heatsinks on the current duals are any indication, it'd take a big, beefy, noisy box to handle four G5s. I'd say it might still happen for a server or workstation, but I wouldn't bet on it yet.
Just for reference, based on my quick testing with my "measly" dual G5, a quad version would draw a HUGE amount of power and have big ol' heatsinks and fans to deal with the heat generated. My old G4 dual drew in the range of 100W, give or take. My G5 draws 170W idling, and can easily go up over 325W if you push it. By extrapolation, a quad version would need at minimum 500W of power, likely a lot more (I don't even have any addon cards, and the computer isn't powering a monitor, either--the Cinema Display via ADC has got to suck some serious juice), and cooling to match.
That adds up to a major power supply, a lot of cooling, a whole lot of heatsink on those G5s (they're probably a third of the volume of the current huge case) or a much louder set of fans/liquid cooling, and one hefty mother of a computer.
Not that it can't be done, but not in the current tower case without at the very least making a massive amount of noise, and I'd guess realistically you'd need something much bigger.