ok there are three options in 4x4 mode. 2WD, 4WD(high), 4WD(LOW).
2wd: Normal driving (city, highway and even rough terrain softroading)
4wd(high): Slippery conditions, steep hills and climbs. Basically for better traction. all 4 wheels are driving and ur car will still be running like in 2wd though fuel consumption may increase.
4wd(low): Heavy offroading. pulling heavy cargo. rough terrains. if ur stuck
heres the general layout of the 4wd gear. down (2wd), slide it up (neutral), slide it to the right(4wd high), slide it up(4wd low). Its better u engage ur main gear to neutral, switch to the desired 4wd option and then change gears. same goes for diengaing the 4wd gear. the 4wd can also be engaged when the vehical is moving at low speed but its better u stop the car.