I finally found the time to test and document my multi cartridge PCB.
It supports EPROMS with up to 1 MB which gives you up to 8 ROM slots (128K games).
(So no C4CPC successor, sorry.)
The PCB can be easily customized to host 1,2,4 or 8 games on a 128K,256K,512K or 1MB EPROM.
Documentation and Gerbers can be found here:
https://github.com/etomuc/gx4000-multi-cartridge
I'll probably refine the documentation a bit over the next couple of days and the shell for the cartridge is still missing but it should be sufficient already to build cartridges.