input code for citroen zx?

My dads citroen zx has had the battery disconnected, and now needs the code put into the stereo, we have the code for it, but i think my dad has tried to put it in before, he reckons there were 4 bars for the code to be put into but now there are only 3, i have left the ignition on hoping that after an hour the code will reset and allow us to put it in properly this time, any assistance on how to input this correctly would be greatly appreciated.
Thanks in advance
Paul McGill