Recently I upgraded my iPhone 4s to iOS 7.1. Please let me know now if my iPhone 4s compatible for finger print sensor.
No, it's still a more a reliable phone in my experience compared to iPhone 5
Only for iPhone 5s and lator
No, only iPhone 5S have the fingerprint sensor
No, unfortunately the fingerprint sensor is a new feature on the iPhone 5s. It's built into the hardware so no amount of software upgrades will add it to the iPhone 4s.