Your own math is off. If he can reach 7'8 flat footed, that means he would need 28" to get to 10'. He would need 32" to get to 10'4". 32" is a pretty high vertical, 7'8" means he has a pretty big wingspan, and I still think you are very wrong that 4" is enough cushion to dunk.
I could only dunk a men's basketball on rims where I could get my wrist to the rim level. I have big hands and can still palm some basketballs, and I needed that much cushion. That means at least 6-7 inches above the rim, Maybe others were more coordinated and could slip the ball over with 4" cushion in a combo dunk/layup move that is close enough to a dunk to get credit for it. I don't see how it is possible.
A basketball has a diameter of about 9.5". So to actually do a clean dunk, you would need to be 10" above the rim to be shooting down with the ball. But most near dunkers will be able to slip the ball over the rim and turn their wrist down on the ball to approximate a dunk that is more like a layup. I did that a few times with a women's ball. I couldn't get high enough or get the timing right with a man's ball.