The C4CPC isn't made anymore by Gérald due to lack of time and exploding costs (x3).

But the Backbit Pro could be a possible replacement. Beware it's more than 100 dollars, but you also have the to buy the corresponding Amstrad CPC+/GX4000 adaptater :

• the adaptater has a chip for ACID protection bypass
• The adapters include a solderless jumper (one jumper is included, and extra jumpers can be purchased, required for ACID bypass)
• 3.5mm jack allows connecting a BackButton (to move back in the menu without touching the backbit pro, some systems such as Amstrad require an included solderless jumper) or a GenAssister
• Supports cartridge images (BIN/CPR) up to 1MB in size
• Real-time-clock remembers date & time even after shutoff. Useful when creating new files or versions
• One-button diagnostic mode helps you identify system faults (on most systems)
• Upgradeable via MicroSD card