I agree with Bri in that if you're feeling ambiguous about buying Sims 4, then maybe you should wait. Several people have also said how they have a lot they haven't done yet with Sims 3. I can understand wanting to get all that you can from that game.
For me personally, Sims 3 has run it's course and I have explored all professions, adventures, futures, etc. So I'm ready to move on and am excited for something new, not necessarily a re-packaged older version of the Sims.
Multitasking is going to be an awesome feature. It's little things that I've noticed. Like when one of the guru's had his sim ask the guy in the gym to come over and train him. Previously, my sims would have had to stop what he was doing, walk over and talk to that sim and then they would go back to the gym equipment. However, in what I saw, he clicked on the interaction on that sim and he walked right over without the active sim having to stop training. This is big to me. I get annoyed if you're in the middle of something and to add someone, you have to stop, walk over to them, talk for a bit and then go right back to where you were before.
To me, it all seems to play out a lot more realistically. Particularly with the multitasking - chatting while cooking - doing things on your tablet while on the toilet (I confess, I've done this in real life) - being able to work on multiple things at the same time will be exciting for me. Yes, I'll miss CASt - however, I also see this as a good thing because I had masses of CC patterns. If I didn't use a mod, it would take a minute just to load one category of clothing for a female. Planning new outfits was very time consuming. So in terms of actual game play, this will likely work out to be a good thing for me.
And I'm happy for some features to be added later. I feel certain that there's going to be so much to do, that having some stuff for on down the road when I'm ready for it will be better. I'm planning on eventually exploring all available base game careers (both branches of each - not sure people realize that all professions have two branches). And then there's finding all the hidden things, getting all the rewards, etc. I've warned all my World of Warcraft friends that September is my Sims 4 month.
We're going on holiday for three weeks in October - queue Sims withdrawal! And then in November, the new expansion for World of Warcraft hits - I'm going to be a busy girl this fall - but very happy!
For those that are on the fence and not sure, stay tuned and see how our stories turn out and what we post about The Sims 4. I'm already planning on a blog-like thread on these forums to document my Sims 4 experience with my first sim. Not a story - more like a journal of me as the player. My plan is to post something for each Sim day - what happened, things I learned, etc. I hope people will find it fun and useful.