I could be wrong, but I'm pretty sure the 4G option under cellular is independent of what APN you use. I have a BUILD 920 but I'm on AT&T. I am using the GoPhone plan and until recently I could not get on the LTE towers and use the LTE APN but I was still able to set my max connection speed to 4G even though my APN was the 3G one.
If you dial ##3282 and then hit the "..." and go into settings, is your network type set to 3G only or automatic?
Also, FWIW, I don't use LTE. Since the BUILD phones never got the 1314 update LTE is pretty flaky. When I do flip it on the data is inconsistent and it kills the battery a lot quicker. I'm hoping that GDR2/Amber will fix that.