What system apps can be safely removed with Titanium backup on Nexus 4?
If your real purpose is to improve battery life, you just can freeze them ($6 dollars spent very well for Titanium Backup). If you don't want to spend money, i think you can use Root Uninstaller Free, that includes "disable" option in the free version: all you have to do is to open it and disable the ones you don't want.
it also includes backup and unistall options if you don't trust. (backups to sdcard/rootuninstaller i think)

you just have to pay attenction to the ones you remove, bacause no official list is findable: just don't uninstall or freeze the app if its name does not convince you like OPP, HTML visualizer, package installer, SIM toolkit,... and so on.

just do a complete backup of your device and, in the worst case, reset your phone.

everything there requires ROOT access