This would be a great startup platform for your project.
You get 4 very similar processors connected. They only differ in their respective I/O devices, but I guess not much work would be needed to allow any of them to take over functions of the other. Now here the shared memory is a vulnerable point, but that too could be changed.

That thing has been done. By some big corporations backed by billions of dollars, with choice of custom parts they only desired, with microprocessors, not microcontrollers and with an army of developers.

Wouldn't it be interesting to reinvent the wheel yourself, with full understanding of its inner workings, ability to modify it at will, with parts you can get in your local store and with specs you want, not some marketing drone half the word away?