Forgive the dumb question, but are you putting the transmission in neutral before trying to shift into 4 low? Most newer vehicles won't shift the transfer case to low range unless it is not in gear.

Outside of that, is the lever making it all the way to low range on the transfer case? The shift rod might require adjustment. Maybe try unhooking the linkage and then forcing it to the 4 low position to make sure the transfer case is okay?