My daughter (4.5 as well) has been rattling on about monsters every night. I'm not sure I believe she is actually scared of monsters..I think she just doesn't want to go to bed...but in any case, I told her that monsters aren't real. She then tells me that if monsters aren't real, then fairies aren't real, so monsters must be real (great reasoning skills this one has)....So I told her that there are no monsters in our house, and the doors are locked, they can't get in. Her response? They have magic powers and can go through walls. When I then tell her that mommy put a spell on the house so that nothing can get in, she called me silly and told me that human's don't have magical powers.
I wish I could help you. Monster spray won't cut it with my daughter. I'm hoping she is just scared of monsters because her daddy has been gone on a long trip. When he gets home tonight, I'm hoping he has a solution.