Hi!,
In a few days i will receive an Amstrad GX4000 8) so,I'm interested in the C4CPC cartridge (with shell if possible).
Where I can write to order one unit?
The C4CPC is currently not available. A successor might see the light one day but there is no official announcement when this will happen.
The Backbit Pro is the only currently available option for a true multi-cartridge solution.
You can also build your own cartridges based on Eproms or even with EEproms. Eeprom based cartridges are easier to reprogram if you want to switch games but it's not as comfortable as a C4CPC or Backbit Pro.