A difference of say 1% with the taller one in front is common. So a 4.10 front and a 4.11 rear will work. Jeep folks claim that you can go to 4%. I don't know what parts would give you that. The slightly taller gear in the front helps keep steering stable when a lot of slip is involved(mud/snow). Any thing bigger than the 4% limit will put way too much stress on the transfer case, or the axles, and that can't be good.
