Well its fairly simple 8) When you will be in low - you will see 4wd stick going seriously close to horizontal position 8)

To do that - just stop (just to be sure). Then start, shift gear into N (while pressing brakes of course), then swiftly push cap on 4wd stick down and with same move - push it towards you and down, then move gear from N to any other...

Then of course do not forget that getting off from 4L to 4H and 2H is a wee bit of process 8)