Where in the Nexus 4 is the wire wireless charging coil?
The charging coil is embedded into the back panel of the phone and is located directly over the battery, as shown in steps 7 and 8 of IFixIt's teardown.
The battery isn't quite non-replaceable, as the above mentioned teardown shows.  It's somewhat involved to replace it (you'll need a Torx screwdriver, a Philips screwdriver, a plastic opening tool, and some patience), but it's perfectly doable.
If you're wanting something that will work for sure, the official wireless charger would probably be your best bet.  However, it's not exactly cheap at $50.  I personally use this charger with my Galaxy S3, which I added a wireless receiver to.  I find it works well and charges right through my lifeproof case.