For Xenia and zoos, the VHOs will be fine. Some of the lower light acros might survive at the very top of your tank, but I wouldn't expect much from them with only 4 VHOs.
MH and VHO produce a different amount of PAR (photosynthetically available radiation, or "intensity") per watt. Because of this, X watts of MH does not create the same light conditions as X watts of PC, T5 HO, or VHO lighting. Within lighting types, different bulbs of the same wattage can produce different PAR. A 10,000K bulb usually puts out more PAR than a 14,000K bulb or a 20,000K bulb.
Essentially, watts is nothing more than the energy consumed by the bulb. The difference between lighting types is the efficiency by which the watts are used to create light. MH, generally, creates some of the highest PAR per watt of the common lighting types. The rest fall in line something like this -- T5 HO, VHO, Power Compacts, Normal Fluorescents.
If you feel you need more light, it may be a reasonable upgrade to keep the IceCap 660 and use it to run T5 HO lamps with individual reflectors. Running T5 HO on an IC ballast is commonly called overdriving. It's safe when done properly (like any other DIY project), but does create more heat and does require some sort of cooling for the bulbs to keep them running optimally. Overdriving T5 HO bulbs does shorten their lifespan some, but they'll still go 12 months or more without needing to be replaced. Individual reflectors run about $20 a piece for the 4'-ers, the bulbs average out near $20 each, and endcaps / mounts can easily be found for $12 - $15 or less a pair (perhaps cheaper if you shop around).
