Go 6 so you can have more bulb combos to play with, as well as more power since you want it to be SPS dominant (and the price difference between 4 and 6 probably isn't that much; heaven forbid you realize/wonder if 4 bulbs isn't enough).
Whatever it is you'll have no problem lighting it either way. The 6 instead of 4 actually will probably help more with spreading out the light back to front than providing more intense lighting for the corals in the tank.
I'd love to throw a few T5s in my tank to even out my LED lighting, but there's a really short supply of 16" bulbs, and it's a pain to retrofit (and I'm pretty lazy).
I'll stop commenting there, because I'm tired of every thread discussing lighting options turning into LED vs. T5/MH. They're all good. You'll really enjoy your tank with T5s or any other lighting option you choose.