I went through the same questions awhile back and also have a C4CPC flash cart. I bought a universal supply from Amazon that I could configure as needed. I use a 9V supply with at least 750mA. Here are a few notes I found from various people on forums:
Amstrad GX4000: 9V 750mA is fine. Just make sure it has a 2.1mm inner pole so you cannot plug it to the 5V input by mistake (that will destroy the GX4000).
- use any regulated DC 9V to 12V 750mA (positive on center pin, GND on outer pin), powering the 11V entry. The 11V is used to power the 5V regulator and to drive the scart switching pin.