4k on a 5.5" screen is a little over 800 DPI, which is a bit excessive. 500 DPI is where print looks pretty much perfect, printers go to 600 because it's double the old 300 de-facto standard. So yeah, a 1440p screen is about the reasonable limit for a 5.5" phone, although 1080p is visually perfect and uses less power for most people.
In fact, previous 4k phones have rendered everything at 1080p anyway, except for video and photos which could use the full 4k.
The only reason I can think of to have 4k on a phone is for when you want to screencast to a bigger display. Maybe Android has some limitation that the casted screen can only be the same or lower resolution than the main one.
The HDR I can get behind, but the 4k resolution sounds like a gimmick.