The reason B&H does not sell the Nikon 200-400 f4 lens is because it has been updated by the Nikon 180-400 mm f4. And it comes with the 1.4 teleconverter built in.
https://www.bhphotovideo.com/c/product/1383763-REG/nikon_180_400mm_f_4e_tc_1_4.html?sts=pi&pim=Y
And by the way, auto focus is slower with any teleconverter.
And by the way, my Sony 600mm f4 is only 6 lbs. is center weighted and feels lighter in the hands than with either the 100-400 or the 200-600, and is sharpest wide open, as all fast telephoto primes are. That's why everyone makes them.
And by the way, the 100-400 is sharpest without the teleconverter, and unlike others on this site, I will offer proof. In the image of the dog below, you can enlarge and read every number on the circular tag on his collar. No teleconverter needed. And the dog was coming at me, usually very hard for a focusing system to get it right, congratulations Sony a9 focusing system.
If you need any help learning more about photography, I offer free guide services to Florida Wetlands. I would be happy to teach you.
Good luck and keep on shooting until the end.

You may have to put your camera in .jpg only mode to see the menu. I do not have an A7R4 or an A9II. My latest is an A7R3 so I'm not familiar with how the A7R4 may differ from the A7R3.

On the A7R4, from the "Help Guide:"

Clear Image Zoom/Digital Zoom (Zoom)
When you use a zoom function other than the optical zoom, you can exceed the zoom range of the optical zoom.

MENU → (Camera Settings2) → [Zoom Setting] → [ClearImage Zoom] or [Digital Zoom].

When a power zoom lens is mounted, move the zoom lever or the zoom ring of the zoom lens to enlarge subjects.

When you exceed the zoom range of the optical zoom, the camera automatically switches to another zoom than the optical zoom.

When a lens other than a power zoom lens is mounted, select MENU → (Camera Settings2) → [Zoom] → desired value.

Press the center of the control wheel to exit.

You can also assign the [Zoom] function to the desired button by selecting MENU → (Camera Settings2) → [Custom Key] or [Custom Key].

I did not check for the A9II

ID, at first you confused me--or I was confusing sensor size with image size... An APS-C sensor (format) size can come in many image sizes (file sizes), such as 24 MP or 40 Mp... so a larger sized image file will take more enlargement, whereas format size will not (necessarily) matter. This is different from film, where the format size itself directly controls enlargement limitations. (Film ISO is sort of like different digital file sizes on a given format.)

If I move up from 21 MP APS-C to 40 MP APS-C would it even matter whether a "full frame" sensor or crop-sensor is used for 40 MP? I ask because my Hasselblad H2 accommodates sensors of the same size, while the MPs are different--and some people prefer the image quality of fewer (but larger) pixels. They say more pixels in the same space increases the relative amount of black space between them.

Thanks the info you supplied previously was good.

I tried it briefly at 2X and was impressed. The image quality was excellent considering it was SOOC and enlarged more than the APS-C mode. I used the A7R4 and Tamron 70-180, 2.8 lens.

However I noticed some serious short comings. It limits the focuses modes to average, no auto eye and related features, no multiple frame rates, no tracking or moving the focus point...perhaps more.

I can't give that up.

Thank you for improving my understanding Sony camera functions.

Thank you for summarizing your findings. When I have used CIZ it has been in occasional situations where I have just needed more reach for my adapted 400mm Canon lens on my a6500 and I wasn't using, or trying to use, any of the features you mentioned that are lost, tracking, eye focus, etc. I definitely lose some features when using adapted Canon lens but fortunately they are features I am not dependent on for the way I shoot and likely wouldn't use even if they were available to me. Thanks again for sharing your experience. That information will be helpful going forward in making decisions about lens purchase and shooting technique.