If you're not too attached to iOS, I've just recently upgraded my old Samsung S3 to a Sony Xperia Z3 Compact. It's a ~4.5" phone with phenomenal battery life. It's got all the power of a flagship phone, but is the only flagship that is <5" that I could find. I've been using it for about 9 hours today so far (since charging) and it's still at 96% charge.
That's the Android phone that usually gets recommended in these discussions. It looks decent enough, especially for Android hardware, but it's basically the same size as the iPhone 6, and the "cost" to me of switching to Android isn't yet worth the benefit of upgrading from my still-excellent 5S. I think it will be another product cycle before I seriously consider any phone upgrade. If I really think about it, I'm probably using a phone less and less every year.
I've heard that durability is an issue on the Z3. I'd love a phone that small which is tough enough not to need a case. (Currently I have an LG G2, which I find too large, despite my large hands; I had to get a case for it after dropping it twice and smashing the screen).
