The Cubit Case from X-Doria fits iPhone 4 or 4S. The body is shatterproof polycarbonate, available in black or white. The back of the case has an open grid that can be filled with colored TPU squares that are decorative and cushioning. The squares can be arranged as desired. Someone suggested this was a good case for parents, because they could pop the case off and the kid could play with and arrange the TPU squares. I think you should reserve the toy mode for older kids, because the TPU squares are a choking hazard for younger kids. The Cubit is $24.99.
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