I have finally been able to order a Compute Module 4 - 8GB memory, 32GB MMC and WiFi 
and it will be delivered on Saturday. I also have an IO carrier board too and have an idea for the board.
I am going to do a little Frankensteining connecting it up to the pi-top[4] just for fun so will look fuggly if it works.
Another thing I would like to look at is make/design a carrier board that mimics the RPi 4 and have all the same IO as the RPi to be able to plop it directly in the pi-top[4] which I foresee a couple issues already with cooling and DSI and CSI connections mainly due to the CM4 design itself.
It’s going to be different working with a CM4 but will also be fun too