A 200-500 f/4 would really be the upgrade of the 200-400, and not a replacement for the already-skipped-over 500mm f/4 II. That would signal to me that Canon doesn't think that focal length deserves a big white prime slot.
I recall on the failure for the 500mm III to appear, the reasoning cited was that the weight improvements weren't enough of a difference for the smaller lens. The primary benefit of the 600mm III over the II was the weight loss (coincidentally, I took delivery of a 600mm III this afternoon to replace the Mark II I'm about to list. I'm, of course, making the upgrade for the weight.)
I'd be disappointed if we didn't see a new 500 f/4 prime, as that was our hope that we'd actually get a new optical design in a big white. We haven't had a fresh design that improved optical performance in more than a decade. I appreciate the Mark IIIs did improve things significantly from a usability perspective. Perhaps I ask too much for IQ improvements. Seeing as Nikon and Sony's more recent designs have roughly equivalent image quality as Canon's probably means they're all designing toward a common set of limits in physics.
Neuro and I have been rocking the same Mark II lens for a long while, both not seeing a good reason to change. I finally fell to the Mark III this week because I found a really good deal on a used one, and I figure now's the time from a depreciation curve perspective. Interesting that he's considering the theoretical 200-500 f/4 as his path. If light enough, that could make sense, especially if it had the internal TC.