I received my top-of-the-range 2021 Apple Mac mini M1 with 16 GB DDR4 RAM and 2 TB NVMe flash 23rd January 2021.

2021 Apple Mac mini 16 GB + 2 TB

1. 8-core CPU
2. 8-core GPU
3. 16-core Neural Engine
4. 1. 16 GB DDR4 RAM
5. 2 TB NVMe flash

2021 Apple Mac Cube

1. 16-core CPU
2. 16-core GPU
3. 32-core Neural Engine
4. 64 GB DDR4 RAM
5. 8 TB NVMe flash

2021 Apple Mac Cube Extreme

Could I take three/four M1 8-core 2021 Mac minis to build a cluster or video wall stack in a single enclosure?

Yes, a Mac mini M1-based video wall cluster is doable.