Does an iphone 4 case fit an iphone 4s

not all

It depends. If you have a Verizon phone now, you will be able to use the case on an iPhone 4s. If you use an AT&T phone, there are a few things you need to watch out. I just wrote a blog about this. Check it out:

https://icollectgadget.wordpress.com/2011/10/09/how-to-choose-cases-for-iphone-4 s-and-iphone-4/

I have a variety of iPhone 4 cases and none fit the iPhone 4S. The mute button on the iPhone 4S has been moved down a bit nearer the two volume buttons so it no longer sits in the middle of its cut out on a case. In fact I think the edge of the cut out may even jam the mute button from working, I dare not try it !!!

I bought a Casemate 'barely there' case last week for a new 4S and it fitted perfectly. Bought another yesterday from same source (Amazon UK) and the case has changed. A mush larger square opening around the lens/ flash has replaced the former teardrop shape and the cutout for the Apple logo has been omitted, probably because of the proximity of the new larger lens/flash opening.

Is there another sensor hidden near the flash/lens?

Comparing the two iPhones, the iPhone 4S has four black strips within the silver edge metal casing, the iPhone 4 has three strips. I cannot see another sensor hidden near the flash/lens. The only button that has been moved is the mute button which is now very slightly down a bit towards the plus volume button.