Rob,
I think I would give T5's a try. I just swapped out 4 110 Watt VHOs on a 660 ballast to 4 T5s on the same 660. They are overdriven to 85 watts, but work great.
If space is not an issue, then I would definitely use 4 bulbs no matter which way you go. The 660 can handle the 4 x 48" with no problem.
Don't forget the fans...
Gary