I don't have a 5th gen, but this is pretty standard across the board. 4low, is LOW. It is essentially the lowest gearing possible and driving in it, at speed, for long periods is a easiest way to fry your transmission and diff. It is meant for extreme low power, usually to pull out a vehicle or to get out of a bog. I have never used 4lo for a continues period. I wheel in 4hi, and only pop into 4lo when I think I will need it. Only for short bursts.
As for speed limit. Honestly, much above 10mph and the RPM is just shooting up. I am not sure of the exact max, but I suggest staying low. If you are traveling faster than 10-15 mph, you don't need to be in low.
As for the m4x4 mileage a month. No, you don't need to drive 16 miles a month; but you do need to regularly exercise the system so it doesn't freeze up on you.