Here's an older post of me asking: https://news.ycombinator.com/item?id=12540268
Z5C (350$) or Z3C (250) seems to come up a lot in my search. I'm also tempted by the nexus 5x (probably at my limit at 5.2" but it's only 250ish), and perhaps the new pixel (which is 5" but probably way more than I'm willing to pay for a phone). Another interesting contender is Nextbit Robin, which is 5.2 inches for 230ish and very pretty.
I've been eyeing Xiaomi phones for a while, but unsure that I want to go to the hassle of getting a device that's unservicable in the US.
On another route, HTC one M8 (5" - 170ish!) seems to be an old flagship that still does well, and us now super cheap. I wonder what other similar ones there are.
edit: to be honest I've given up on finding sub-5", and thinking about 5.2.
The 2013 Moto X was perfect: flagship specs, 4.7 inches, and basically stock Android. Seems such perfection was a flash in the pan.
This being said, I recently discovered the "one-handed keyboard feature" on my OP2 and absolutely love it. I can finally answer messages while swiping with a single hand.
Screenshot: https://ibin.co/2xCShGbpGdGt.png