6 bulbs would be much better. Does you fixture have a parabolic reflector or ICR (individually contoured reflector)?
If a parabolic reflector, it's worthless. The key to T5HO is the individual reflectors or ICR, that wrap around each bulb, in a ' gull wing' configuration.
4 bulbs with quality reflectors, will allow you to grow some SPS, as long as they're kept high in the tank.