I believe you can go to the 4120 on your Rev C BUT I have a Rev H and started on 3970 and went to 4120. I'm not saying it will happen but with that firmware I was getting map zoom issues. For example, every time the car was started after an overnight period etc the default zoom would be different. Sometimes it would go from 100yrds, which was my preferred view to a mile. The next time it would be 3 miles. The last time it did it, it had zoomed out to 70 miles!
I was advised to go to 5274 which I have and although it is better, it still does it but not to such extremes.