Upload & Sell: Off
I preface this by saying that I have no experience whatever with any of the 400/2.8 lenses, but I have been using a 400 DO and a 500/4 together for quite a few years.
It may well be the case that the 500/4 has better image quality than does the 400 DO, but, in my experience, I'd have to be looking at 100% crops in the screen to see it, and I have never had any inclination to compare 100% crops. When I look at prints taken with the two lenses, the image quality is so close that I have to look at the file information to remind myself which lens took which pictures.
Over the years, I've become firmly convinced that discussions on internet forums would make one think that image quality differences of different lenses (and camera bodies) are much greater than they are in actual usage. Several times, I've taken medium-sized prints from both lenses and showed them to people who had claimed, without ever having owned one, that the image quality of the 400 DO was unacceptable. I asked them to compare the prints and to tell me which ones were taken with which lens, and I have yet to come across a single person who was able to accurately tell me which lens took which pictures at any level of accuracy higher than what you would expect with random guesses. I've also done the same experiment a couple of times with pictures posted on this board. Each time, those who guessed actually ended up with accuracy much lower than what you would expect from random guesses.
I've also had experience using and comparing images from the 400 DO and the 300/2.8 IS. My assessment would be that the bare 300 is a bit sharper than the 400 DO, but the 300 plus 1.4x (thus, 420/f4) comparison was pretty much a wash, and images from the 400 DO and 1.4x were noticeably better than those from the 300/2.8 and a 2x. In addition the AF with the DO was faster than that of the 300 with a 1.4x.
As soon as I got my 400 DO about 6 years ago, it immediately became my favorite lens. For the wildlife shooting that I primarily do, it is a joy to use. When you take off the tripod ring, it barely weighs a bit over 4 pounds, and being able to use a lens of that size and weight allows me mobility and opportunities to shoot hand held for extended periods of time that using any of the bigger, heavier lenses wouldn't give me. And that allows me to get shots that I otherwise would not get. It is the lens that I choose to use in any situation where its focal length is sufficient.
Out of all of the different pieces of equipment, both bodies and lenses, that I own, it is the single piece of equipment that I am the least likely to ever sell. The only thing that could get me to part with it would be if Canon ever introduced a 400/f4 DO II that was even lighter, and the likelihood of that happening is, I think, quite slim.
I'll try the same experiment that I described above here. Some of these images of an eagle fledgling were taken with my 500/4 and my 1D Mark IV, and others were taken with the 400 DO and my 7D. If anyone cares to guess which photos came from which camera and lens combination, I'll be happy to tell you how well you did.