A simple search of the "Nexus 4" sub-forum will yield several choices, but mine was Multiple Device Backup Battery.

 

I personally use the Anker SlimTalk 3200mAh battery. It has a built in micro usb cable so you don't need to carry one with you plus it also doubles as a flashlight which is pretty nifty. But there is also a full size usb port on it so you can charge pretty much any device with it if it doesn't use micro usb. It gives me a full charge when my battery is near empty (sub 10%) with 25% or so left over and is about the size of a credit card so it's easy to carry around with you.

 

I second that recommendation as I use the same one and it is great.