I have the HTC One Mini[1]. It is small with a nice screen, camera and battery life. It isn't going to impress anyone but I can use it comfortably and keep it in my pocket.
Honestly I'm hoping for a 4" screen or smaller. Maybe it doesn't exist. And though the screen is 0.3" smaller than the Z3 Compact that ZenoArrow mentioned, the One Mini is taller and thicker, and only slightly narrower.
In the same ballpack though, so I'll need to do some digging and find out how the cameras and all that stack up.
[1] http://www.htc.com/us/smartphones/htc-one-mini/