What are the *technical* reasons preventing connection to 4G? How can I diagnose myself?
While I'm not intimately familiar with Sprint's 4G or your phone (sorry) I have found in my own CDMA troubleshooting that the built in HTC diagnostic tools can be very useful. I found this guide over at XDA as one of the better lists of Sprint CDMA diag codes (even though it's not for your phone specifically). I recommend trying ##33284# or ##3282#, one of these should bring up a utility that lists very specific information about your cellular network connection.