The good news is that the battery – a 2,100mAh lithium ion polymer cell made by LG, can be replaced. The bad news, however, is that you won't be able to do that on the fly. That's because the cable connecting the battery to the circuit board is held by a couple of screws. Moreover, removing the smartphone's back cover also requires the use of a screwdriver as a pair of torx screws are holding the plate in place. As a side note, the antennas for Wi-Fi, GPS, and cellular connectivity are embedded in the back plate.
So yeah, the LG Nexus 4 battery can be replaced if needed. For example, if its performance drops significantly over time, one can pop in a brand new cell. However, doing so would require a bit of tinkering, yet nothing that can't be done at home with tools available at your local hardware store. Of course, if you aren't confident in your skills, seeking help from a professional is advisable.
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source: ComedyMonger (Reddit); Images taken by @nerdtalker from Anandtech