I have an original developer BUILD phone with T-Mobile USA service.
The label used to show H, but I needed to flash ATT firmware after bricking it.
Now it shows 4G, but it is probably HSPA+ since the download speed is about 8mbp.
I really didn't do anything special after I flashed the firmware & restore the backup from skydrive. The data connection was initially showing E, but after I got wifi connected & restored apps it was 4G. May be I made a phone call to make sure the phone still worked. That was about it.