First, thanks for all the responses. It hasn't made it much easier to decide what I'll do, but I appreciate the time you've all put into this.
Second, a bit of background. I've been using an iPhone since 2010, a Mac since 2001, and until Yosemite I very much enjoyed both Mac OS X and iOS.
Third, I used an iPhone 4 from 2010 until 2014, I bought an iPhone 6 last October. A few days ago my wife dropped her iPhone 4S and I gave her my 6, which is why I now need to decide on whether I get her 4S repaired/replaced by Apple for 200€, or whether I buy a whole new phone for myself.
Fourth, until Apple announced that iOS 9 would run on the 4S, we had expected to buy a new iPhone of some kind when the new models are released later this year. But iOS 9 will run on the 4S, even if some developers decide to drop support for the 32-bit A5 and A6 SoCs.
Fifth, I plan to replace my 2010 MacBook later this year with a Hackintosh. Though I intend to use El Capitan as my main OS, I will use Windows 10 to play games, and so buying a Windows Phone now doesn't seem so ridiculous as it did before.
Sixth, the 4S has 32GB storage, and for how I use an iPhone that really is the minimum. I've had to offload all my music save for a few playlists, where my 128GB iPhone 6 was able to store it all. An 8GB or 16GB phone does not seem reasonable for my use.
Seventh, though the screen is cracked I've been using the 4S (running iOS 8.3) for the last few days and other than Crossy Road and the camera, it has been satisfactory for all that I do with it. The iPhone 4/4S design is by far my favourite, much more so than the 6, the fact that I can keep it in my pocket and sit down much more comfortably is an added benefit.
Eighth, I don't trust Google, so an Android phone seems unlikely for me, but the Lumia 640 does look very nice. It reminds me of the iPhone 5c, which I would also consider if they sold a 32GB version, but it's currently only avaiable new with 8GB.
Ninth, the Apple features I use that I would miss on a Windows phone are shared photo streams, Safari iCloud tabs, contacts and calendars. I would need to look into alternative solutions that work between Mac OS X and Windows Phone and are not Google. Does anyone know of good options?
Tenth, thanks again for your feedback and suggestions and for reading all this too.