I have the same problem. I'm Homesharing using an old Mac mini running iTunes 12.8.x. I can watch via home sharing on all gen 3 AppleTVs, a gen 4 AppleTV, and via iPhone, iPad and Macs... but my AppleTV 4k does not connect. However, it does show the latest/most recent movies that have been uploaded on the Home Screen when on the computers app. Clicking on the Computers app though just gives a cannot connect and make sure your computer is on the same network (which it is).
I've tried updating to the newest firmware, updating iTunes (now most current), rebooting routers and access points, etc. Cannot get it to view the library.
Any pointers would be greatly appreciated. I'm getting to the point of trying the beta software to see if that solves it.
Thanks,
DJ