I'm loving my DR-70D. Here's my take on the Fourth XLR conundrum: I made an aluminum cradle with a chassis-mount XLR attached, held via the bottom 1/4" thread.
The 4th XLR connector is transferred to the opposite side via a shorty male XLR and 6" of balanced lav cable that runs across the rear of the unit. It adds about 1-1/4" to the depth, but still easily fits into almost any bag. I also removed the flimsy rear door cover, making the battery hatch and SD card slot more accessible. (Obviously I've no interest in using the onboard mics! 8>]
cheers
Dave