Playback works well - up to 60fps. However, export to H.265 creates a lot of Swap. Rendering went with 15-18fps. All videos on a SMB network drive but the GPU was the bottleneck for rendering.
Swap was even around 24GB with 5 videos which I tested first. Using 4x4K it went 9 GB before stabilizing at around 2GB. No effects or grading whatsoever - plain 4K60 SDR videos.
One single SDR 4K clip renders to 8K at 25fps. Using Superscale 2x makes that 0.5-1fps.
For 8K rendering you may be better off with 32GB RAM minimum or trying the M4 Pro model maybe with 24GB. For 4K/6K editing the base 16/256 M4 Mac mini seems sufficient when all video storage will be on external drives or network.
Edit: added single 4k->8K rendering performance