I had just recently fixed a broken 4G on my phone so there are a few links that proved helpful. Basically what it boils down to is that if your MAC address did not change in any of the processes that might have broken the 4G, then there is hope. Apparently, if the MAC changed to a generic MAC address, you're kind of hosed. If not, I wiped a tree.xml file using adb and let the phone regenerate it's MAC address. It has worked for me. I also flashed the newest radio and Wiimax files as stated in the thread below. Follow the original post and then jump to post 157 for the adb instructions in resetting the tree.xml file. It took a lot of reading and it boiled down to this.
[GUIDE] Bad WiMax MAC? Broken 4G after update? Fix HERE! - xda-developers