I've never tried anything but muscle cars in the drag but most likely the level 4 parts bring the power to levels beyond the available traction and you lose precious seconds struggling for grip.
Agreed, but modern muscles (i.e. Shelby GT500, Corvette C6) have adequate traction to cope with all this power. They start with high level 2 traction, and can reach low-to-mid level 4 traction when upgraded.