[RoadiJeff]

I've read that if you can see the pavement then 2H. Otherwise, 4H for snow covered roads. You can go as slow or fast in 4H as you can in 2H although I wouldn't recommend it if the roads are slick. If you can go 55+ mph the roads probably aren't all that bad and thus you'd be in 2H.

I don't know about 4L. It came with our Jeep but I doubt it will ever get used. I guess it's for going slow off-road or something.

 

[RoadiJeff]

You might want to check your manual since it specifies to be moving.

Yes indeed. Pg 431 of the 2013 owners manual:

4H to 4L or 4L to 4H

With the vehicle rolling at 2 to 3 mph (3 to 5 km/h), shift an
automatic transmission into NEUTRAL (N), or press the
clutch pedal on a manual transmission. While the vehicle is
coasting at 2 to 3 mph (3 to 5 km/h), shift the transfer case
lever firmly to the desired position. Do not pause with the
transfer case in N (Neutral). Once the shift is completed,
place the automatic transmission into DRIVE or release the
clutch pedal on a manual transmission.

NOTE: Shifting into or out of 4L is possible with the
vehicle completely stopped; however, difficulty may occur
due to the mating teeth not being properly aligned. Several
attempts may be required for clutch teeth alignment and
shift completion to occur. The preferred method is with the
vehicle rolling at 2 to 3 mph (3 to 5 km/h). Avoid
attempting to engage or disengage 4L with the vehicle
moving faster than 2 to 3 mph (3 to 5 km/h).