Looking forward to it.
After the signal bars came up on GSM I switched to LTE / CDMA in both the testing menu, AND in the settings, mobile networks menu. Then I rebooted, the 4G logo came up shortly after, for me it configured my network mode to LTE / CDMA / EvDo / GSM/WCDMA after the reboot. If it doesn't work the first time check to see if it reset your configuration to GSM, if so try again.
Others may have gotten it other but similar ways. But keep tinkering and you'll get it.