The 4k (4026 x 2169) Apple ProRes 422 Proxy coded movie has a bitrate of 146 MBit/s at 25 FPS, measured on the MBP. The same movie rendered as 4k MP4 (h.264) has a bitrate of 55 MBit/s at 25 FPS - roughly one third of the ProRes file. Call it imagination, but the Apple ProRes movie looks clearly better on the ATV than the h.264 movie. And especially for timelapse movies, the quality of the footage is essential ("milky way at night"). So I still wonder, what these movies will look like when rendered with a higher quality Apple ProRes codec. Unfortunately I did not find a way to display the bitrate in the VLC player on the ATV. VLC is the only player able to play the Apple ProRes movie smoothly.
But I ordered the new ATV with the A12 Bionic so I will see, if the ATV is the bottleneck as you mentioned or if it's the 1 GB ethernet / NAS combination (what I assume). I will update this post with the result.