Now my dad has a problem with his G4. He can't send messages just says sending all the time. He's tried turning the phone off & on again & now luck. My only guess is that his memory is full & the phone is old but what else could cause this?
Storage being full wouldn't stop messages from sending - it might stop him from receiving messages.
Have him bring the phone to his carrier and ask them to get his account working. (They'll probably have to remove the account, then enter it again, if the droid he's talking to doesn't know how to fix it.)
If he's not reveivingmessages also, check the APN. (Settings > Mobile date)