Source: https://developer.ubuntu.com/en/start/ubuntu-for-devices/installing-ubuntu-for-devices/

thinkpad ~ # adb devices List of devices attached 0079d583c50fafcc device

thinkpad ~ # adb shell grep ro.product.name system/build.prop ro.product.name=occam

thinkpad ~ # adb shell grep ro.product.device /system/build.prop ro.product.device=mako

thinkpad ~ # adb shell grep build.id /system/build.prop ro.build.id=LRX22C

thinkpad ~ # adb reboot bootloader

see bootloader screen on mobile device

thinkpad ~ # fastboot oem unlock
…
OKAY [ 32.456s]
finished. total time: 32.456s

boot into bootloader → volume +/- PLUS POWER

$ ubuntu-device-flash touch –channel=devel –bootstrap
2015/03/16 18:41:12 Expecting the device to be in the bootloader… waiting
2015/03/16 18:41:12 Device is |mako|
2015/03/16 18:41:12 Flashing version 16 from devel channel and server https://system-image.ubuntu.com to device mako

Download:

Recovery: https://twrp.me/devices/lgnexus4.html
OS: https://download.lineageos.org/mako 
Google Apps: http://opengapps.org/ go for pico

Clean Installation:

factory reset in old os
adb reboot bootloader
fastboot flash recovery your_Downloaded_Image.img
go to recovery
adb push $os /sdcard/
adb push $gapps /sdcard/
Wipe > Advanced Wipe > /System + /Data + /Cache Confirm "Swipe to Wipe"
install $os
install $gapps
reboot
enjoy