Two to four inches are great for week old chicks and older. If you see them huddling together too much under that for warmth like they are too cold, you can always lower it. The temps that are under the pad aren't as important as the temps directly against the pad, as that is the area they are coming in contact with to gain warmth. As long as they can come in contact with the surface of the pad and the pad is warm enough to keep them comfortable, I'd not worry about the exact temps in the brooder space.