When plowing driveways, you are constantly doing a lot of jockeying around out in the street (bare blacktop) mostly. If you use 4HI, you can just pop in out of 4x4 and into 2HI on the fly and not bind up out in the street. While if you used 4LO, you have to stop and shift into 2HI. Whenever you see your tranny or engine struggling with deep snow, etc. Use 4LO. It will depend alot on what your rear gears are. If you run 3.42's you may want to use 4LO more than a guy with 4.11's.
Hope this helped,