i'm a little hazey on this subject but i dont think there is a gen4 3S-GE i think after gen 3 there was a VVTi engine or something like that and then i think it went to the 5S-FE
i have my haynes manual here and the engines it covers are...
2S-E
3S-FE
3S-GE
5S-FE
4a-FE
7A-FE
granted this is only a celica manual but does give a rough idea on how the engines went.
but back to your original question i dont really know how to tell when the generations of 3S-GE's changed but i'm pretty sure there wasnt a 4th gen
feel free to point out any mistakes i may have uttered in my rambleings