What Diesel means is your engine power goes to the transmission, to the transfer case, and then through the driveshafts to the differential (pumpkin) on each axle. An open differential (like most stock diffs) will apply power to the wheel with the least traction. This is good on-road cause you're less likely to spin out, but it sucks offroad. If your axle has a limited-slip differential or a locker, it will apply power to both wheels on that axle at the same time.

 

Putting it in 4 doesn't add any power to your rear wheels. It only engages your front wheels in addition to your rear wheels (which are still receiving the same power as in 2) Everyone has said it: IF YOU DISCONNECT YOUR FRONT DRIVESHAFT, 4x4 WILL DO NOTHING. NOTHING. NOTHING. BECAUSE YOU HAVE NO CONNECTION TO YOUR FRONT WHEELS. IT ADDS NO EXTRA POWER TO YOUR REAR WHEELS.