Grow Light - how much better is 4 than 2?

[ladyslipper]

Hi guys, this may seem like a dumb question but why do we need four grow lights? I am using 2 48 inch 40 watt grow lights that is perpendicular to an east/north east facing window. I am growing mostly phals (both species & hybrids) with a few other kinds mixed in.

The other question is do grow lights come in different wattage (like 60 or 80 watts) and would this burn the plants (none of my flowers are close to the bulbs)

regards mary