The Principle for 3-Factor authentication is
Something you have (like a smartcard) Something you know (like a pin) Something you are (like a fingerprint)
Then what is the 4th factor. Is it like 4D :)
I don't think 4-factor authentication is a well defined concept - it'd just be called "multi-factor".
Probably somewhere you are! You can only log in if you and device are in a certain location, for example.