Is IP networking possible over a Thunderbolt linked Mac mini M4 cluster?

You can use Thunderbolt as a point-to-point network, yes.

You’ll need to run the Macs as IP routers, and Macs aren’t great IP routers.

Or with more common hardware and networking, get 10 GbE NICs and a 10 GbE switch.

As for clustering, Apple dropped Xgrid and related support long ago. And Xsan support for Fibre Channel ended more recently. Which means not much native support for clustering. This as I usually expect clustering to include remote or shared storage access.

Routing: https://www.ms8.com/mastering-network-routing-on-macos/

You can do this. By default it’s a layer-2 bus style topology. We have clusters of Studios and Mini M4s, where we daisy chain them…then the last one back to the first one to make a full loop.

Assign each Mac an IP from the same network to the TB Bridge Interface, but a different network than the Ethernet connections and no gateway. Easy. One Mac can reboot and only that one goes down.

Works great with exos, resilio and syncthing for faster lateral networking. It can also find each other for Minio replication and Content Caching Peering.

What are you trying to accomplish by doing this? And what management app would you be using?

FWIW, I suspect that cascading TB connections that way is not going to work the way you hope it might.