Must the Nexus 4 be rooted in order to install custom ROMs? Do I need to root my nexus 4 in order to install custom roms such as cyanogenmod etc or just unlocking bootloader is enough as i have flashed my nexus with ubuntu 13.10 by just unlocking bootloader?
You don't need to root to install ROMs on a Nexus 4. Just unlocking the bootloader and installing a custom recovery (Such as TWRP or CWM) through fastboot is all you need to start installing custom Android builds.