Passcode is required to enable Touch ID
With iOS 9, Apple updated the list of times when iPhones will require a passcode rather than Touch ID. The list is now:


Restarting the device
Five failed fingerprint recognition attempts
Receiving a remote lock command via Find My iPhone
Enrolling new fingerprints in Touch ID
Not having been unlocked in any fashion in 48 hours
NEW! When the iPhone or iPad hasn’t been unlocked with its passcode in the previous six days, and Touch ID hasn’t been used to unlock it within the last eight hours


You probably hit the brand new last condition without knowing it. I know you said your phone was charging for only 4 hours, but depending on how long it had been since you actually unlocked, it may still have happened.

Source