I wonder why the 400 mm DO f4 + EF 2x III extender is not as good as the 500 mm f4 + EF2x III extender?
I compared these two directly against each other a few years ago when deciding between precisely those lenses. I went with the 500mm f/4 mostly because I got a really good deal on a used one. The 400 DO II was newer and not as discounted among the used inventory at the time.
The image quality results showed me that the 500 was very slightly better with the TCs, particularly in the bokeh quality (so, you known, hair splitting). But if I'd had gotten a better offer on a used 400 f/4 DO II, I'd have probably taken it. I really liked the size better.
In retrospect, now shooting the 600 f/4 II primarily, the 500 was the better decision from a focal length perspective, but the IQ issue is still pretty even.