If you use a gear that is almost as large as the wheel's diameter, then of course you're going to have very little ground clearance. Two factors to always consider: The wheel will always dip into the carpet a bit, and the field is never perfectly flat, so there will be some bumps and ridges. Try using a 13:70 gear spread. The 72T gear has an outer diameter of 3.7" (radius of 1.85"). The 4" wheel has a radius of 2". That means on a perfect surface with a perfect setup you should have 0.15" of ground clearance, which in competition condition is still very little. Combine that with about 3/32" of carpet deformation and about 1/8" of bumps and ridges on the field (those two numbers being very generous guesstimates), and you'll find yourself with negative ground clearance (aka your gear touching the carpet).