One bulb will not do a 4' tank unless you would be willing to accept the two sides to be in near total shadow and the middle to be insanly intense with light.
For 4' long tanks you should spec two MH
bulbs both mounted about 1' in from the sides. This will split the difference on the center brace and any shadows that the brace might cast will be negated by the opposing bulb. A single bulb setup over the brace would also cast a shadow into the tank if the brace is not totally transparent.
Unless you was going to go for a full SPS
tank you can get by with 175W mH
bulbs over the 75. When doing MH
you should match the ballast wattage with the bulb wattage. I like XM brand bulbs.