Good question, and one I asked an intel officer shortly after becoming
mission
qualified in the BUFF. Here's the way he layed it out:

1st Generation (early jet fighter) - MiG-15,17,19 or F-84, F-86

2nd Generation (early supersonic)- MiG-21 or Century Series

3rd Generation (advanced supersonic) - MiG-23 or F-4

4th Generation ("next generation") - MiG-25, 29, 31, Su-27 or F-14,15,16

5th Generation (???) - MiG 1.42, Su-30 series or FA/22, F-35

Good reply, but I don't think I'd classify Mig-25 as 4th generation. Its
avionics and relatively primitive aerodynamics (I know, mach 2.8 - 3.2
primitive aero? Well, the X-2 was quite primitive too) don't match the 4th
Gen aircraft.

R / John