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...