Battery cases are big. This is, of course, because they are cases that contain not just your phone, but another battery with which you will charge your phone.
And so here we give you a quick look at the Mugen Power Case for the Nexus 4. It's a big battery case — 4,500 mAh and easily double the thickness of the Nexus 4. But it's also simple to remove (without you having to worry that your phone will go flying out. But unlike most battery cases, in which the phone slides onto the microUSB plug somehow to get its juice — Mugen opts for an external plug. It (mostly) tucks away into the soft-touch case when not in use. When you need that extra power, you unstow it, connect it to your phone and pray nobody sees what you've done.
This battery case isn't all that much of a looker. It's decent enough with its soft-touch finish and blue LEDs, but the external plug is as homely as it is functional. That's just a simple fact. But what you do get here is a basic 1A in/out battery case that'll protect your phone (everything but the display) and give you a far more power than the Nexus 4's 2100 mAh internal battery can manage.
You can snag a case directly from Mugen for $89.50.