The flip case on my 4s is from Proporta, aluminium front and back, covered in lightly padded leather. Fantastic quality and makes it feel pretty damn safe! Excellent after-sales service too. The stitching came out of my first one and I'd lost the paperwork for it and it was a present from someone I couldn't get the details from but I called them up and they accepted it no problem and even replaced it with the next model up.
Pricey though - £30 ish - but it's very good quality and the service was second to none.
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edit: -
this one