The iPhone 4s has a 32-bit processor and cannot be updated to iOS 10 or higher. Because Apple switched from 32-bit processors to 64-bit ones a long time ago, you might not even be able to access the App Store – and if by some miracle you could, you'd find that most developers do not make apps that run on iOS 9 any more.
The iPhone 4s uses 2G and 3G cellular frequencies and does not support any 4G or 5G ones. In the U.S., phone companies have shut off their 2G and 3G networks and the iPhone 4s would be useless as a cell phone. I believe that other parts of the world are following suit – recently, Australian carriers shut down their 3G networks.
So it might be time to consider replacing that old phone – a phone introduced over 13 years ago (in October 2011).