SimGuruLyndsay confirms The Sims 4 has no planned End-Date.

I mean, I like Sims 4 just fine, but it's still limited by its base and could've been so much better if things had been planned better from the start. TS5, assuming it's done well, would be appreciated

I'll keep playing as long as there's interesting content, but if something better comes along before TS5 comes, I would probably switch over