I click with Steve's opinions and teaching over Thom's. I just do not click with Thom for whatever reason. I spent $ on Thom's Z9 guide book and it just does not fit me. I started reading, then skimmed through to see if it was all just not my cup of tea and it is not. So I quit wasting time on it. Others "really" resonate with Thom. We are all different in the way we learn etc.. and that is not a bad or good thing it just is what it isI’m also curious about Thom’s comment not to sell the 500pf for a 400+TC.
Compared to the F-mount 500mm f/5.6E PF, the 400mm f/4.5 VR S with a 1.4x teleconverter (560mm, so not perfectly comparable) seems to do slightly worse in the center, slightly better in the corners. For most telephoto work, sharper centers is what you want, so don't throw that 500mm PF away just yet ;~).
Finally, the 400mm f/4.5 VR S is a little better with a 1.4x teleconverter than the 100-400mm f/4.5-5.6 VR S, but both are quite good. In the F-mount world I was not a fan (or user) of teleconverters. In the Z-mount world, I'm coming around to liking (and using) them, at least the 1.4x version.
I do agree with Thom and his thoughts on the Z100-400 and as I noted I have that lens because I want the variable focal length. I did not do any formal testing with the "Z400 f/4.5" and 1.4 TC and only a few sample simple test shots before my wife adopted it. I sold the 500pf after I saw how it stacked up against the Z100-400 so I did not have it to do a at the same time and same subject test against the Z400 f/4.5 with 1.4 TC.
I just came back from getting frame filling ID shots on some rare for this area sapsuckers with the Z800pf that I could not have come close to with cropped shots from 500pf, Z100-400, Z 400 f/4.5 or the 600 f/4E I sold last year.