Go for it...
Upgrading from 5 to 4 is all about experience. They (USCF) wants to be get used to bike in close quarters with others and sharpen your bike handling skills. IMO, cross racing requires more skills than road racing. Go for it...
It's ultimately up to the regional rep. but it doesn't hurt to try. I get the impression that they rules are just...rules. If you are willing to write an upgrade request, they'll accept it if you are close to what is required.