Try cleaning it. The average A4000 keyboard is over 10 years old now, and if it hasn't been cleaned in its entire lifetime, it must be very dirty.
Use a set of fine mechanic screwdrivers and try not to lose all the little screws. Clean the main PCB, but gently, in warm water. There is probably a lot of dirt on the contacts below the keys. Ear cleaning tips or a sponge will do nicely, just make sure that you no cotton or fabric gets stuck. Let it dry thoroughly.
Put all the lose keys in a tub of warm water and some soap, stir around and polish a brush or sponge if necessary. Do the same for the plastic casing.
If it still doesn't work well, it may be worn out. Try to re-coat the contacts on the circuit boards with a pencil, it has worked for me.
PC keyboards require a small adapter to be used, since they use incompatible signals.
