You want a HPS for flower, not a halide. A 250 MH is a GREAT light for vegging and is best suited for a grower who intends to flower under a HPS600.
4' of space is not much. You want 6'. 3 gallon pots stand about 14" high, then add the depth of the hood on the light and that leaves you with only 2' of grow space between soil and bulb. You need some space between plant and bulb even WITH a cooltube (much much less but still..) so you might think about a pure-flourescent grow in that space. Or LEDs... lol! Procyon seems to be the only system of demonstrated merit on the market atm though...