You use 4hi when traveling on slick roads. You can shift from 2 high and 4 high at anytime below around 50 mph. You'll notice it march harder to steer when in 4 high.
You should not shift into or out of 2 low without being stopped and in neutral. Transmission works the same except much slower with more grunt. This should be used to go very slow in mud or deep snow. Also good for going super slow if pulling someone out of a ditch. You should hear a big clunk with shifting in and out of 2 low.
