It sounds like something is blocking your proximity sensor at the top corner of your phone. I don't know where exactly it is for the Nexus 4,but on the Galaxy Nexus it's the top right corner of the phone, but to the left of the front camera; it's to the right of the earpiece.
Do you have a screen protector or a case on your phone? Was it, or were they, perhaps put on incorrectly, mistakenly covering even slightly the proximity sensor up top? I find the proximity sensor to be somewhat sensitive on android, though I prefer it over a dud one. It's supposed to turn the screen off when the phone's too close to something, whether it's your face, or even if a finger gets in front of it for a mere .05 seconds when on a call.
It may sound dumb or too basic, but that's my suggestion. When on a phone call, the sensor's sensitivity seems to increase substantially. Though like I said, it's kind of a good thing usually. Check Google where your sensor is and see if it's somehow blocked during your calls.
Sent from my Galaxy Nexus using Android Central Forums