Fitting 2015 4K screen into a 2017 4K system?

Thank you for taking the time to address the matter Dan. Very helpful.

Take care.

FYI--I have done the opposite, using a 2017 4k display in a late 2015 iMac and it works, once you exchange the eDP cables (using the 2015 cable with the 2017 screen). The iMac passes the basic Apple Diagnostics after the screen exchange. "About this Mac" shows a retina display with 4096 x 2304 resolution, under System Report>Graphics Displays. However, when clicking the "Displays" tab in the first "About this Mac" window, it shows a retina display with 3840 x 2160 resolution and the scaling in Display preferences is for the 3840 x 2160 resolution. Nonetheless, the screen looks perfect and acts perfectly, and the resolution can be manipulated with third-party apps, if you so wish. (Or by going back to Mojave with the option/click on "Scaled" in the Display preferences.)

Any insights or comments?

@kaellis - I find it hard to believe the cable alone allowed the display to work.

The issue here is the display standards between eDP and iDP which is one of the differences between the two. I haven't made a big study on compatibility between the two.

@danj Yes, just changing the cable alone, and the display works. I was also surprised. I also just updated the computer to Catalina, and option/scaled still brings up the actual resolutions on the iMac, which I thought was not available in Catalina (does not work on my MBP in Catalina). I may have not made it clear but both are Retina 4K iMacs and I believe both have eDP cables, at least this is how they are described when buying either of them. I really appreciate your comments and help in figuring this out.

@kaellis - The info I have shows eDP on the 2015 model and iDP on the 2017 (both in question are 4k displays).

What you are implying is the cable is keyed in some fashion.