Ok, this isn't a Bird in Flight, I know the difference.
But this Female Cheetah is running down a Thompson's Gazelle at somewhere between 80kph & 100kph, I'm no BIF photographer in any way, I leave that up to People like Gary Samples who does an excellent job of it, but I don't think too many BIF are any more difficult to Photograph than this situation. And I don't for one moment say that this Image is an amazing Cheetah in Flight Image, but the 200-400f/4 does this amazingly well and in this Image you have the disadvantage of Tall Grass to get in the way of the Lenses ability to focus on the subject, not something you generally have to worry too much about with BIF.
To shoot this with the 100-400f/4-5.6, you would need the 1.4x converter attached and then your shooting an Animal running @ 100kph @ f/8, not an impossible task I admit but a lot more difficult than it seems, but the 200-400f/4 in this instance was on the 1Dx 560mm f/5.6 1/2000th ISO800 and just handled it supremely well, I don't think the 100-400 could have done it as well, but, I didn't have the 100-400 so it's conjecture, based on experience, mine.
Could the 100-400 have done it, I'm sure it could have, I'm just glad I had the 200-400 instead so I was more sure of getting The Shot.