I write from the perspective of someone who played TS1 when it came out and only rediscovered the Sims franchise a few months ago. Since then I've played both TS3 and TS4. Now, I almost always play TS4.
Like many of you, I enjoy the art style of TS4 very much. The Sims are realistic without being "uncanny valley" realistic. I enjoy being able to create an almost unlimited variety of characters and bring them into my game. I do use a few CC hairstyles and some CC clothing, but it's all Maxis Match, because I like the game's out-of-the-box aesthetic.
I'm not a good builder, but I love browsing through the Gallery, looking for lots I can use. Lately, I've been playing in a Simworld that was entirely created by @SweetieWright_84. I have other saves that take place in other settings, and I can choose any of these stages on which to play the game.
I mod my games very gently, and I'm glad TS4 supports modding so well.
And, finally, I can create a world where things usually turn out just fine. Right now, I enjoy escaping for a while into a place where I can focus on the good in people, a place where the only chaos and upheaval is comic in nature. I tend to create a lot of kind, happy Sims, and when I add some "villains" to the story, I can make sure the good Sims win in the end.
I know that's not how the world really is, and I know some of you prefer worlds that have a more realistic feel...but that's what I like best about TS4. We can create the kinds of worlds we want, and enjoy having our Sims live in them.