The choice of which longer lens you might like best depends a lot on what you shoot and where.
I have the Z 100-400 mm and the 500 mm PF. Have not at this point ordered a Z 400 mm f4.5. I have an 800 mm PF on order, as I do a lot of bird photography, but who knows when that will come. The 500 mm PF has been my most used lens for wildlife since I got it, although the Z 100-400 mm is starting to catch up.
I think the 500 mm PF is a bit better optically than the Z 100-400 and can get you to 700 mm with a 1.4x TCIII. And I am sure that the Z 400 mm f4.5 is better optically at 400 mm. All 3 lenses would be nice in a kayak or canoe, where I do a lot of photography. But the versatility of the Z 100-400 is very valuable to me. And I like the close minimum focus distance for small (but not tiny) subjects like butterflies, dragonflies and frogs.
I have used the Z 100-400 this year on my Z9 in Yellowstone, Florida, the Khutzeymateen, Iceland and in my kayak in northern Minnesota, both with and without the Z 1.4x TC. I used the zoom feature a lot. For example, in the Khutzeymateen, we shot from a zodiac in water channels in an estuary. At times, the grizzlies were quite close and at other times, they were farther away. The ability to go from 100 mm to 400 mm without changing lenses and without reaching for another body was very nice. The image quality that I get from the Z 100-400 (including with the Z 1.4x TC) is very good in my opinion. I think I get some shots that I might otherwise miss changing bodies or lenses.
I have thought about getting the Z 400 mm f4.5, but for me it would be in addition to the Z 100-400, not in its place, I can see times when a kit with the Z 70-200 f2.8 and the Z 400 f4.5 would be very nice for lower light. But for now, I think the Z 100-400 mm, 500 mm PF and the 800 mm PF (when I can get one) will be a great kit.
Thom Hogan has an interesting comparison of the telephoto options for a Z shooter.
https://www.zsystemuser.com/nikon-z-system-news-and/telephoto-options-round-two.html