My workplace want to change everyone's start time from 5am to 4am, can they do this?

I work for a supermarket, stacking shelves from 5am to 10am, four days a week. It's a small team of about 8-10 people. The manager has informed us this morning that they want people to start having everyone in at 4am to 9am so we can get everything done earlier. I am not okay with this, neither are the rest of us. I think that they'll likely bin us off if we refuse to start an hour earlier especially as the turn around is so high and a lot of us are still on probation. What is the likelihood of us actually getting fired for this? Will they go through with it? Has this happened at your workplace before? I'm stressing as tbh, the job market is tough and we all have bills to pay but starting at 4am is just simply not feasible for me, especially as I signed a contract for 5am - 10am!!!

Thanks :)