I would suggest designating a clean up captian for each center each week. This way you can give the responsibility to the children and they can motivate the others to help them. Post their responsibilities and make sure their is a visual for the class jobs. Also make sure you are giving a signal for clean up time. This could be an oral warning of 5 to 10 minutes and an auditory warning by playing "clean up" music. This seems to work well with the 4 year old's I have worked with.