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There is at least one other multiple-CM4 board 'in development', though I can't share any detail, other than keep an eye on this page for when it might appear: https://pipci.jeffgeerling.com/boards_cm

Not affiliated with the project in any way, but I do know this one is launching soon! https://turingpi.com/

Edit: Spoke too soon, only works with the CM3/3+. Leaving it here in case anyone is interested.

They have a v2, thanks to the user who commented after you! https://turingpi.com/v2/

https://turingpi.com/turing-pi-2-announcement/