Interesting. However...
Actually, the latter two look like they are bordering on oversharpened to me... That might be because of the conversion to internet resolutions, uploading process, of course.
Color rendition and background blur are certainly nicer in the last two, though I suspect it might be due to different lighting conditions. First two appear to be in full or nearly full sun, judging from the shadows and directionality... While the bottom two appear to be under moderate overcast or even in light shade.
Not defending the 400 DO (I don't have/use it or the f2.8, so I don't have a horse in this race)... just commenting.
I have seen some really nice work out of the 400 DO. Mainly, I think it's been people who needed as long a lens as possible, but left with no choice but to hand hold it most of the time.
I wouldn't be at all surprised if somewhat different post processing methods were needed... More sharpening, or different settings for example.