Hi J.,
My daughter who is 5 has started some separation anxiety when I take her to preschool. I have a home daycare so part of it for her may be that she's having fun at home and doesn't want to miss out on anything going on at home to go to preschool. She always has a great time there and it's just been the transition of drop off that's been an issue.
I am told by the teachers that this is a common thing at this age. Kids learn that they are separate from us and that bad things can happen to us and to them independently. So thoughts about this causes the anxiety.
Does your daughter have a good time at daycare after the initial drop off problem?
Also, you may check into another environment for her that will provide a little more age appropriate activities and involve more kids her own age. In my experience a daycare environment usually has a range of ages but 4 1/2 tends to be on the older side. Even though your daughter may have friends there, if they are not her age she may not be challenged in the way she wants or needs to be.
So there are some thoughts... the suddenness of the situation would make me think that something happened to trigger her reluctance to go, but it may be some other change that you haven't considered that's playing into it too. Check in with her daycare provider and see if she's been talking about anything new... kindergarten, a move, death, all of these things could cause the anxiety.
Good luck!