So let's talk about the 400 f/4.5.
Improvements compared to the 500 PF:
- Closer focusing
- No FTZ required
- Much quieter
- Sharper images (MFT's are comparing f/4.5 to f/5.6 and not f/5.6 to f/5.6. Of course a f/5.6 lens is sharper than a f/4.5)
- Creamier bokeh that is less fussy in direct sun (Easily fixed on the 500 by moving a step to your left or right)
- No constant VR noise
Improvements compared to the 100-400mm f/4.5-5.6 S
- Requires half the ISO on the same subject (Suggesting the 100-400 is a little darker than f/5.6 or the 400 f/4.5 is just brighter than the 4.5 suggests)
- Sharper images
- Creamier bokeh
Improvements compared to both:
- Physically smaller
- Focuses much quicker (no slow down on the Z6 and fc)
- Fits in my bags better, even ones where the 500 wanted to stick out
- Gathers a lot more light, more than the f/4.5 suggests
- Has the weight closer to the body so feels lighter
So because I am not a professional reviewer, that's about all you can get from me. I however do note people say the 500 is sharper, but the 500 is a f/5.6 lens and the charts are being compared f/4.5 vs f/5.6 and not f/5.6 vs f/5.6.
In all I am delighted I sold my 500 PF, the 400 f/4.5 handles much better and that matters way more to me than 100mm (I have my 1.4x in my pocket if I need more), and it fits in a smaller backup. It even fits in my shoulder bag.
A downside that it shares with the 100-400 and 500 PF is the lack of lens lugs for attaching your strap to instead of the camera body. But it is so light that this seems irrelevant.
Another wee note: Somehow the shorter and less chubby 70-200 f/2.8 S feels heavier than the 400 f/4.5.
All in, I can see the 400 f/4.5 being my walk about lens for going about forests where the 800 is too dark. I am also considering canceling my 100-400 order and 400 f/2.8 order since in the first hour the 400 f/4.5 proved itself in low light of a dark wooded area that only gets lighter as we go into the winter with less foliage. Needing too high a ISO on a fox would likely change my mind there. The 100-400 only wins me over for versatility and macro, but I have a 70-200 f/2.8 and 105mm macro f/2.8 so I will need to try out the scenario where I rented my 100-400 before.