Stay there until after September 1. Then tell her that she agreed that you could move in and stay without pay. If she says no, ask her to show you lease, invoice, evidence that you agree to pay for those four days.
And show her your lease that you can actually live at your old place in those four days.
If you want to stay there long, then you got to be careful.
And I forget to mention, you can just say : " Are you kidding? Everybody move earlier or later for a few days during the move at Montreal, I've never known anybody can get key and move without some kind of agreement. Do you allow people to move into your home without your permission or lease?"
My suggestion would be, just be straightforward to her.
Tell her that, since she informed you to move in earlier without saying anything about the rental fee, so by common sense you thought these four days were free.
Tell her that you are still paying your rental fee for that 4 days to your previous landlord, so you really hope you won't have to pay double, you're willing to stay long and it's only 4 days...
Try to deal with her with calmness, and in a right, negotiating attitude to get problem solved, chances are that you will get it.
guy, i think this landlord tried to screw u up, but nothing to worry. Anyway, the worst consequence is that you pay her four days of rent. However, if i were you, i wouldn't pay her now. if she asks about rent again, you answer politely, " sure, i will move in September 1, you give me your keys so i move some goods in" What can she say? don't argue. She can do nothing against you. Bingo! simple!