Congrats on your success so far. Sounds like you have already unlocked your device, so the first thing you should do now is update your recovery image, perhaps to something like TWRP2. You download the latest .img file and then boot into fastboot mode (
adb reboot bootloader from adb or hold down the up (?) volume key while booting). Then use fastboot on your computer to flash the recovery .img file into place with the command
fastboot flash recovery openrecovery-twrp-18.104.22.168-mako.img or whatever. (Or, alternatively you can use a .zip and use the existing recovery to essentially flash the zip to update over itself)
Now that your recovery image is up to date, it's time to get a cm-12 nightly, which you can do here. Put the resulting cm-...NIGHTLY-mako.zip on the phone's internal storage, then boot into recovery mode (you can do this from either the reboot menu, from
adb reboot recovery in adb, or by powering up with the down (?) volume held down). Now use the menu to do three things: 1. do a factory reset to wipe everything, 2. wipe the /system partition to get rid of all traces of cm10, and 3. Flash the cm-12 nightly .zip. If you want to add a step to add the google apps (gapps) I'll leave that to you to discovery.
At this point you can reboot and should be in the nightly. Note that you haven't updated your radio in this process. The radio would be wiped if you did a stock update from your carrier, but since you're updating with a cyanogenmod nightly, the radio update isn't included so should not be touched. But if you want to double-check, verify that there's no
modem.img in the zip that would update the radio.
If at any point you DO want to update your radio, check out this archive of Nexus 4 radios, including some with LTE enabled and/or disabled. You can flash these radio updates with your recovery just like any other zip.