Last month I installed two 8' units with an electronic ballast (less energy consumption and operates below 60 degrees w/o issues) in my 2 car garage and I'm so glad I didn't go with the four 4' units. If I didn't have to cover the holes from the two single light fixtures, I would have gone with a single H.O. unit as recommended by Rick. I'd also recommend covering the tubes with a plastic sheath or go with safety bulbs that are coated in silicone. It's a little overkill, but with my luck something would tag a tube and it would come crashing down on a car. As far as height, I don't have a specific rule to follow, but I think 12' to 14' off the ground would give you the best light disbursement.
Speaking of overkill, there are two sights you might want to check out that might shed a little more light on your question (sorry....couldn't resist the pun):
Lithonia Lighting - The best value in lighting
Visual - Interior Lighting Tool