I'm also interested in both 3 and 4 and have been for a while now, but never bought either because I had the same question. Which is better, 3 or 4? I've heard it both ways. 3 is better than 4. 4 is better than 3. I personally liked 1 better than 2.
What I would suggest doing is watching a few hours of Youtube gameplay videos for each game. This may help you to decide. If not, go ahead and buy 3 and play it for a few months. Then buy 4. They are not too pricey since it's been several years since their release date. Addons for each will cost more money of course.
The Sims 3 (plus Seasons expansion) is about $27.
Google Search: The Sims 3 Plus Seasons
(Doesn't say it supports Win 10 but it MIGHT. Check around first).
The Sims 4 (plus 2 expansion packs) is about $32.
Google search: The Sims 4 Bonus Exclusive Bundle - PC Game
(Read reviews. It comes with an empty disc case. Ridiculous. You will have to download it and use the code they give you).
Make sure they will run on your system. Look at the minimum requirements for each. I've heard they will run on Win 10 but they were not made for Win 10, so you are taking a chance.
If you pick one and try it out and it works, let me know.
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