There's 4wd then there's "true" 4wd. Now on some machines including (but not limited to) your RTV you have a 4wd system which does NOT include a means to lock both front wheel. What you are left with are 2 wheels which can always get power (the back ones) and 2 wheels which under normal conditions get power (the front ones). Now when one of the front wheels meets resistance of any sort all of the power is diverted from that wheel and sent to the one with little resistance. If say you are climbing a steep hill and hit a slick patch or are just barely able to keep traction, once traction on one wheel is lost (in the front) it will spin and the other will have 0 power. On things like the new HPX's and the XUV you have a locking front differential which will lock itself once slipage occurs.