For me, I think TS3 served its purpose for the time it was in. When it was out, I was completely invested. Loved the open world and I really liked the supernatural pack the best & I loved the houseboats plus the horses. I tried to go back to it, but just couldn't get into it....and even on a gaming laptop, my computer can barely run the game without constant crashes.
I don't have a desire to go back to TS3. But without mods, I couldn't play TS4, whereas I never needed mods for TS3 or even TS2. So I think that alone says a lot that TS4 just doesn't have enough 'meat' compared to the other versions. I've always felt that the next iteration was supposed to be a step up from the previous iteration and there might be some things that don't carry over, but generally the next iteration is supposed to be "better". I've always felt that TS4 was a step down from TS3 or TS2. The enhancements or new things in TS4 didn't overshadow all the things that was missing. It was hard for me to go from having babies in TS3 that my Sims could put in strollers, carry around, put in play pen etc. to having baby objects in TS4. My list can go on, on the things that didn't carry over, that made TS4 base game seem lacking. In addition, I think the emotion system, memory system and just overall depth is what TS4 lacks. & it's taken way too long to get basic NPC's which I believe should have been there from the start such as the firefighter. Still missing the burglar.
However, on the plus side, I do think TS4 has the best toddlers, vampires, CAS, and building system. & with my heavily modded version of TS4, I get what I need that I feel is lacking (story progression, emotions/jealousy reactions, memories, etc.) I have fun with the game.....I feel that I'm able to create some super creative and dynamic stories that I wasn't able to in TS2 or TS3. From what I recall with TS2....the game was actually really, really challenging, so I didn't have time to create a story, I was trying to survive lol. & TS3, I just played, really wasn't trying to create a stories or do challenges. So I don't know.....it's a hard one for me, to compare TS4 to the other versions. I can totally see both sides....of what is missing in TS4 and what is good.