Can you put an iPhone 4S in an iPhone 4 case?

On websites they always sell the iPhone 4 & 4s cases seperatley and I wanted to buy one online, but they all just say iPhone 4. Just want to make sure I for sure can just in case!

Yeah

Of course, the appearance of iPhone 4s and iPhone 4 are same.

Yes you can, the dimensions of the phones are exactly the same.

Some cases can use in common, but some can't. You should pay more attention before you buy it. I found a site have many good cases can use in common, you can have a look.
http://www.case2case.net/iphone-3g-3gs-cases.html

Yes most of them should fit, it depends on the type of case you have. Pelican cases are hard cases and they are covered by a lifetime warranty. I bought my Pelican case at this website
https://www.pelicanproducts.us/c.Pelican.Micro.Cases.html

Http://www.case2case.net/iphone-3g-3gs-cases/iphone-4g-cases.html
these cases fit your iPhone 4 or iPhone 4S. They are available to both.
hope helped you.