First off, bear in mind that I am using the definitions I've used professionally with the USAF. So, a European or Chinese designer might use different criteria.
Here we go:
GEN 1: MiG-15, F-86
-subsonic jet fighters
GEN 2: F-100, MiG-21, F-102
-supersonic jet fighters
-limited AAM carriage
-radar introduced
GEN 3: MiG-23, F-4, Mirage F-1, MiG-25
-limited look-down/shoot-down performance
-introduced BVR AAM carriage
GEN 4: F-15, F-16, MiG-29, Su-27, F-14
-BVR AAM carriage
-advanced digital avionics introduced
-look-down/shoot-down weapon systems are common
GEN 5: F-22A, F-35
-stealth
-integrated advanced avionics
4.5 GEN aircraft are 4th GEN fighters modified and incorporating 5th GEN features, such as advanced avionics or weapon systems. Examples here are the Su-35, EF-2000, and Rafale.
Here's a decent overview:
http://www.aerospaceweb.org/question/history/q0182.shtml