Not to but heads with anyone but my impression of 4wd is this. In my brothers jeep manual it states there is a part-time 4wd option and a full-time 4wd option. The part time option is to be used offroad or on slippery pavement(never on dry pavement); whereas the full-time option allows the driver to drive the truck in 4wd on all types of pavement without any negative effects. The difference being in part-time the driveshafts are turning at the exact same speed and in full time their is a mechanism that allows the drive shafts to spin at two different speeds(an example being making a turn).