4th button doesn't work?

No-one seems to have an answer to this one. When I replaced the battery after charging, all 4 buttons were working. After the son in law had a go, it might be this, it might be that. It locked out. Now when I try to input a code, 3 buttons work ok but as soon as i press the 4th button i'm locked out again. I've been waiting for the flashing bits to come back but it's getting me down now. Am I doing something wrong or does it need to go in the bin???