I'm a huge fan of Sims 2 and have not played Sims 4 yet but I can tell you theoretically how comparisons can be made based on things I have seen and confirmed:
Sims 4 Wins over Sims 2 in the following:
1- Graphics & more beautiful surroundings
2- CAS
3- Build Mode
4- Travelling system between worlds
5- No. of loading screens are less to start the game.
Sims 2 Wins over Sims 4 in the following:
1- Life stages are more polished
2- More services available over the phone
3- Ability to create your own world right from base game
Things to be determined as I have not played TS4:
- How emotions affect gameplay in TS4
- Leaving for work: Sims 2 have carpool and TS4 don't. Though TS2 sims take time to get in while TS4 sims are faster in vanishing. This I will decide with game play which I like better.
- Careers: TS2 had more normal careers, TS4 have less normal careers, but I'm very keen to see how the promotions work.
- How well the animations are done in TS4 compared to 2
- How smooth TS4 plays and how fast the loading screens are compared to TS2
- How story progression affect rotational play in TS4
Overall TS4 seems to have nice upgrades over TS2 but the game has to be played to know many of the other things.