I have been lurking in here for quite a while but I finally have a reason to post!
You can lock it in 4G only but you will not have voice service.
Open the phone dialer and dial ##program (##7764726) and hit send.
When prompted for the password, enter "000000" (six zeros)
From the Programming Menu, select "06 Test Mode"
Hit "Next" at the bottom 2 times
You will see an option for Network Mode. From that location you can select LTE only.
When you return to the home screen, it may cause your phone to reboot. This is normal.
IMPORTANT: DO NOT CHANGE ANY OTHER SETTINGS! YOU CAN SERIOUSLY MESS UP YOUR PHONE IN THIS MENU!
To change back, follow the instructions again and return the setting to "CDMA, EVDO and LTE"
I have tested this in an LTE area and as soon as I leave the LTE area, my signal disappears and indicates no signal. Hope this helps.