[KenWittlief]

yes and no- if a wheel leaves the ground it will also spin freely,

which means it will draw far less current, your battery will be less loaded, and therefore there will be more voltage (power) available to the other three wheels

probabally not 25% more power, but it will be a significant amount.

Usually in a shoving match the wheels mnaking contact with the floor start spinning anyway - once a wheel starts to spin it doesnt matter how fast it spins, the force it applies is the same. Having twice the HP on a spinning wheel gets you no increase in pushing force.