I'd go with a six bulb set-up. You will have better bulb combo's with the additional lamps, plus you will be able to keep pretty much anything as long as you are using good individual reflectors. 4 might be ok for SPS if you overdrive them. I have four (uvl 12K, b+, b+, aquablue) over my 75 on an icecap 660 with two vho super actinic and the combo is awesome and very bright.