No need to wonder about it. Physics has already figured it out. It's called the inverse square law. A simple way to look at it for what we care about is...

(far distance/near distance) squared = brightness

So for Mike... (18 / 4) squared = 20.25. So by lowering the lights to 4 inches from 18 you increased the light intensity at the water surface by 20.25 times.
Thats why you should stay in school kids