You normally drive in 4x2 on the road, and switch to 4x4 off the road or in snowy or rainy conditions. Wranglers have a transfer case which allows you to switch it from 4x2 to 4x4 by means of a lever, meaning it's only a part-time 4 wheel drive. Never engage 4wd on dry pavement or you will damage your transfer case. And yes... you do get better mileage in 4x2 but it wouldn't really matter since you can't use 4x4 in normal driving.