Having shot side-by-side for a few weeks now, in general the 400 4.5 is
slightly sharper, a tiny bit more clarity/contrast, and with 2/3 stop more OOF rendering than the 100-400. It’s not as big of a difference as one would be led to believe though; aside from the bokeh/blur, shots from the zoom can nearly be made to resemble the prime in PP. Things being almost equal, I’d choose the zoom over the prime for the versatility.
With the 1.4 TC, they’re pretty much equal, which I was surprised /wrt to the 400. As I mentioned a few posts back, it falls well short of the 500PF, which in my eye had better IQ in every regard. This was slated to be my 500PF replacement, but if given the choice, I’d easily take the F-mount lens.
Other thing with the prime: if you’re shooting wide open near MFD: amazing blur and sharpness, but not enough DOF to even cover a small songbird (eyes in focus, maybe the front foot, barely anything else), a shot which for me will go right in the bin. So, best stop down, in which case the 100-400 makes more sense, again.
I don’t know, part of me says to sell the 400 4.5, but another part actually really likes the lens. I think if I were a mammal or large bird shooter, it’d make more sense, but I mostly feel the zoom does 90% of what the prime does. Maybe as a 1-lens travel solution, but doesn’t a zoom make a ton more sense in that regard?
They’re all great pieces of glass, I’d love to have them all

Might keep the 400 a bit longer and see how often it gets the nod over the 800 and 100-400, though I feel it’ll be more a case of “you know, let’s take the 400 out instead, just to change things up”.