4 feet is good enough to lit the tank but if you gonna go with a 5 feet you can do a custom fixture if you are creative enough. 4 feet looks fine on mines your MH will make up for it and also if you go with 3 MH heat can be problem. But then you just gonna have them hanging. And plus you want to have the VHO's to create the dawn and dust effect. So it won't be such a shock to the corals when all the MH shuts off.