4" to 6" is not enough to clear speed bumps, or the difference in a ramps angle of assault, and would cause stopping to lift just to clear the path of travel much more often.
I guess in your case it might be a 6 inch rule....
you might get 4 inch deflection on the tip of a fork of a 10,000 capacity truck when carrying a 4'x4'x 8' stack of 2 by fours and stop suddenly.
This is ONLY to be used to report flooding, spam, advertising and problematic (harassing, abusive or crude) posts.