It'd be easy for me to say TS2 is better than TS4 for one simple reason: the sims were more unique with better personalities and AI that actually made much more sense. Further, whims were more varied and specific to each sim. The memories system, likes and dislikes/personalities, made the sims dynamic as well as more challenging. Another thing that helped to add a sense of realism and even drama was that, in TS2, the sims' relationship bars could be (and were) often different, even for two sims in relationship to each other. It seems crazy that in TS4, we've lost so much of those amazing aspects that lent so much to an incredible simming experience.
That said, TS4 offers so much that TS2 didn't have: the calendar system, the ability to save rooms/lots as well as move them (I remember painstaking time spent moving rooms around grid by grid in TS2). TS4 also offers better and more CAS options (though, for the life of me, why give us so many outfit choices when the game is programmed (not glitched) to change said outfits to something we've not, nor can, edit? why not pull in one of the outfits we've created for our sims? smh).
There is a lot I miss from TS2. Not just what I've mentioned above, but all of the other little touches that made the game a bit deeper and more immersive in general. TS4, however good, feels like a lot of fluff compared to TS2. But, I've not gone back. TS4 is good enough for me for now, though I'd love to see TS5 take the best aspects from previous series to give us well-thought out and fully-rounded game play. I kind of doubt that'll happen though....