I have now used the new four channel feature to make a short test video combining the stereo mics of the camera recorded into a wave file on its card, and a binaural mic recorded onto the video file. When editing, this enables me to crossfade from the binaural mics to the internal mics to avoid the odd sound of my own speech recorded onto mics in my ears.
My hearing is pretty shot these days but the result seems very acceptable. Most of the ambient sound is low level and distant but it comes across well.