For TS4 -
LuchADORE <3
Dat routing fix ((Ahhh mmm))
More life to Sims
Much better Animation
NO SEAMLESS OPEN WORLD <3
Better family interactions
More content ever shipped with any base game
More advanced CAS and Build Mode
Better graphics
Emotions as part of gameplay
Friendlier to Mod and CC
DETAILS which are too many to name! <3 ((Like showers doing things besides raising hygiene, the whole "unsatisfying woohoo", etc.))
For TS3-
A color wheel
((I can't think of anything else))
Now the TS2 to TS4 will be lots harder to choose between. Right now sliding more towards TS4 but will have to play it before I make the final call.