So you are going with a reef with the 40 huh? LOL Figures....
I think the 4 bulb should be fine with that depth, in fact I have the same dilemma with my 20g, I have only about 11 inched of water after the sandbed is accounted for. I'm leaning either towards a 4 bulb T5 setup or a cheap halide setup.
I always recommend ATI bulbs, kove them to death! I would be going with 2 aquablue specials, a blue plus and either another blue plus or a purple plus.
Just me and my preference for blue I guess.