Corals need more blue light than anything. In my last T5 setup I didn't even use a 10k bulb, I hate them.
For 4 bulbs go with 2 ATI Blue Plus, 1 ATI Purple Plus and 1 KZ New Generation.
For 6 bulbs go with 4 ATI Blue Plus, 1 ATI Purple Plus and 1 GE 6500k.