4th radio code button doesn't work(2)?

No-one seems to have an answer to this one. When I replaced the battery on my Transit van after charging, the radio locked out. at this time all 4 buttons were working. 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 30 mins to an hour with radio ignition on 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???