You need modern apps, HTTPS browsing (most websites crash on Mobile Safari 5.1), or day-to-day reliability. Even WhatsApp legacy has shut down.
If you are already on a later version (like iOS 9.3.6) and have jailbroken your device, you can use CoolBooter . downgrade iphone 4s to ios 5
Downgrading an iPhone 4S to iOS 5 is a complex process because Apple no longer officially "signs" this firmware version You need modern apps, HTTPS browsing (most websites