Frankly, I'm just not seeing excellent images from the 200-400mm: most images posted on the Internet taken with this lens could have been taken with a 100-400mm costing a tenth of the price and a fraction of the size/weight. The latest generation of Canon 300mm, 400mm, 500mm and 600mm lenses, in comparison, deliver images that sparkle. If you need focal length versatility, go for a long prime and mid-range zoom. I'd be happy to be proved wrong!
Hi Mikea, you wouldn't be AvTvM or Michael in another life would you ?? I mean, your 3rd Post on CR & you come onto a Thread specifically set up for people to discuss the merits & or handling of the 200-400, post their Images etc, and dump on the Images & the Lens in question, not bad for a 3rd time Poster.
But, if we treat your Post with a modicum of respect, yes, almost all the Images you find taken with the 200-400f/4 could be to some degree emulated using the 100-400f/4-5.6 with a 1.4x converter (200-400f/4 of course isn't able to emulate the 100-400 at less than 200, clearly), I disagree though that in right hands you are going to get anywhere near the same IQ from the 100-400 that you will get from the 200-400, and having owned (and given away) the 100-400 & currently own the 200-400, I, unlike yourself, speak from experience.
If your serious (and I doubt you are) about wanting to discern the IQ difference between the two, it's simple really, hire both & go out and take some Images side by side & see if your prepared to make the financial commitment of the 200-400 over the 100-400, it just seems to me to be a smarter method of determining the Pros & Cons of each Lens prior to the commitment, than reviewing Photos on the Internet.
Fortunately for Canon there aren't too many people out in the real World that share your opinion, currently the 200-400f/4 is basically a back order almost anywhere you can find the Lens, and that's at US$16k compared to the 100-400f/4 at what ?? US1.6k, I wonder why all those Wildlife & Sport photographers are prepared to pay the huge premium to own the 200-400 over the 100-400, Oh yes I know, they clearly have no idea about how best to select gear, more money than Brains, didn't listen to mikea.
I've seen very little on the web re the 200-400f/4 being the ideal Lens for BIF Imaging, I've seen a load on the Web about the 600f/4, 600f/4 + 1.4x, 800f/5.6 being the ideal BIF lenses. And my own experience supports this view, the 200-400 is an amazing Lens for Wildlife & I imagine Sports, but for BIF it can't compare to the 600f/4 + 1.4x, and I own & use that Combo as well, so again, I speak from experience.
Maybe instead of spending your time on the internet reviewing those Thousands if Bird Images you might take some time out & go take some Images yourself with the 100-400f/4 & the 200-400f/4 (Hire them for a few days) & Post them here so we can be convinced you have any idea of what your talking about, I mean this only in a respectful manner of course, I'de love you to be able to convince me, I'm really not interested in convincing you.