4wd means more traction....thats it.
it was ONLY brought into rallying because with about 300bhp through the rears, its just spins and spins and spins....or in Ari, Vatanen own words justa spiiinning and a spiining the wheLLs".
So, with 4wd they could double the power and still have traction on loose power. On tarmac, 4wd, if there is any heavy FW bias, will push on and understeer like a dog! It doent handle as well as a rwd car because you can balance it, it will drift then straighten and snap....no matter how much trackery you are running, jsut a result of having power though the fronts. FOrd experimented with a AWD F1 car back in the days, but it was pants and didnt make it into any races.