Chosen Solution

Can I use an iPhone 4 battery in an iPhone 4s ? Thanks!

Unfortunately the simple answer is no. The iPhones 4’s battery has a different voltage and the connector is slightly different. If you need a new battery you can get one from here, iPhone 4S Battery that will work no problem in your phone. Hope this helps.

Batteries are different physical size as well. The 4 is bigger than the 4s. Normally the 4s will fit in the 4, but not vice-versa.

Unfortunately the simple answer is no. The iPhones 4’s battery has a different voltage and the connector is slightly different.