Slider, once you have completely ruled out the wiring, is there any way to swap your 80's with a set of 8 ohm mains with a buddy for a day or so? This will address whether the Sony is choking on that 4ohm load or not. Also, in the almost imposible case that there is something wrong with one of the 80's, it will show up with the other receiver.
In the past, it has also been mentioned here that the tolerances between thermistors/thermodiscs vary wildly. You may get one receiver that shuts down pre-maturely, and another one of the exact same model that does not.
