a 3hp uses a 3in coil, has a huge topplate weighs about 40lbs, uses 3 3/4 ferrit slugs. and is a very old motor. very common motor...used on eclipse ti (modified) soundsplinter rl-s (modified) audiopulse axis, tc3000, lighting audio x2s, and a few others....the 4hp is a much larger motor. uses a 4 in coil weighs 60lbs plus. is used on ti pros, 5200, and a modified version is used on the cactus sounds mk1,trf, t3 tsns. and I think the hp part is just a name for it.