If you're only contracted to do 4 hours a week and nothing says you have to do extra hours over Christmas, they cannot force you to do overtime. You'll just have to stand your ground and tell them straight that you aren't available and can only do your set shift. In retail this kind of thing happens all the time, they sneak hours on you without asking first and you have to tell them you can't/won't do it. If they are a decent manager they should be able to sort it, if they are incompetent they will have pinned all their hope on you doing those extra hours and have to scrabble around to find cover, which isn't your fault or your problem.