Will iPhone 4s cases fit an iPhone 4?

I ordered one on eBay, but the option was like iPhone 4s/iPhone 4, I couldn't choose which. I don't have an AT&T one. I'm assuming there's no external differences? Serious answers only please.

Hi we interchange cases at home. However we're changing to the 5 series. This means all new cases again. I put a link from apples site to validate your question.

Thanks
Chuck

http://www.virtualphonenumbershop.com