I have decided on 2 row, but obviously more fixtures. My thinking is that I want even light, and don't see the point of adding more wattage to a single point. I'm no lighting expert, but it doesn't seem like you add that much more usable light when you add more bulbs.
2 row is probably lots. I've got 3 rows of 2-bulb units (18 fixtures total) in my 28x28. Ceiling in 9' and painted white
The fixtures I used have 2 rows mounted in them, but can be upgraded to 4 by adding another ballast and more lamp ends.
I used both, but I bought my fixtures used. If I was doing it from scratch, I would go with 2 rows. I tried to stategically hang the 4 row lights, so it works o.k, but the 2 rows I screwed right to the ceiling.