actually,you aren't putting power to all 4 wheels in 4wd unless you have lockers or a limited slip axle in the diffs. In stock form with open differentials, you only have a 2wd because only one tire on each axle is pulling at once.

 

Well no that's not entirely correct, when all 4 tires have equal traction they are each getting 25% of the available engine TQ so all 4 wheels are pulling. If the axles have traction aiding differentials then that balance stays pretty much the same in varying traction conditions.. depending how good the diff is. But if the diffs are open then a single wheel can see anywhere between 0% and 50% of the available engine TQ, and it will be the wheel with the least traction that gets the most TQ unfortunately so we end up with only two wheels pulling in these low traction conditions.