This must vary from phone to phone. I played a video stream from my Slingbox to my phone for 45 minutes after reading the OP to see what my experience would be like. The back of my phone was warmer than standby, but not warm in any meaningful way. Also, the "warmth" was evenly distributed across the entire back surface of the phone; I did not perceive it to be hotter near the camera lens.
Excluding potential phone-specific issues, there are other things you could take into account that might negatively impact this scenario, such as temperature where you are (inside/outside), whether the phone is in a case, whether the phone is being charged on QI charger (not likely for G4, but possible), and other factors that might have an impact. For me, I was sitting at my desk with the phone propped up against a plastic surface, 73 degree room temp, and no case.