Hi, having a problem with 4k buffering. Gig wired ethernet from hp win10 i7/32gb pc to 4kappletv. On the 40gb mkv uhds, blurays etc, no issues at all. On the 70gb mkvs there is about 1min then buffer, bit more playing then more buffering and so on. I just setup share i think smb, didnt make any tweaks to the setup.
Try the SMB version settings changes mentioned. If those don’t work you could try sharing through NFS or use plex to stream them to the Apple TV. SMB has some network overhead which could be causing your issues.