I had a similar problem. I opened the case and found the contacts on the circuit board that the rubber feet of the key touch when you press the key down. I took a strip of wrapping foil about 1.5 cm by 0.5 cm and stuck it with sticky tape over the top of the circuit board contacts being careful to leave the foil abit raise over the contacts themselves, so that when you press the key down it pushes the foil onto the contacts. failing this (as it did for me eventually because the foil gets pushed to the contacts with use) I eventually got an old A500 and used the keyboard from it, which fits perfectly in the A4000 keyboard case.
