Honestly, I'm usually one to get more than I need, so I would buy the 4-bulb to be on the safe side, rather than find out later I didn't have enough light.
Another option would be to look into the TaoTronics LED
's on ebay. They are pretty inexpensive, like $130 each, and according to a few reputable people on here that have them, they are able to keep SPS
with them, so low light corals like shrooms
would be gravy, and you'd get the shimmer effect ~ all for less money than a nice 4-bulb.