I don't have any information regarding renting 4 wheelers but my guess is no as I have been hanging around this message board for almost a year and haven't seen a thread or any information posted. However, you should rely upon the experts for that type of information.
As far as the map....
Try this one. Mata Chica is pretty far above the cut. I've never stayed up there but this should give you a fairly good idea. From what I understand it's pretty difficult to navigate the "road" above the cut so a lot of people use water taxis to travel to town and back.
You might try doing a seach of the messages for "above the cut" or "north end" it is AMAZING what you'll find.