On my aquarium, which is 30" wide, the 8-bulb is just barely enough to have proper front to back coverage. I think you'd be cutting it close with 4 bulbs, unless you raise the fixture up high.
Since I have a 48" tank with a 48" fixture I can't comment on side coverage, but given that the T5 reflectors are designed to reflect front to back, I would suspect there would be considerable shadowing.
Either way you'll have to raise the fixture up high.
Assuming they make a 36" with 8 bulbs, that might work out better. I definitely think a 48" with 8 bulbs would be a total waste.
Get the exact dimensions of the 48" fixture, it might look OK on a 46" tank.