The Qt High-DPI support process allows setting a scale factor (via platform plug-ins, a user environment variable, or potential per-screen configuration files), layering changes to accomodate scaling, QWindow and other platform changes, etc. The HiDPI support is of course centered around new monitors that have very high pixel densities (Retina MacBook Pro, many smaller 4K displays, etc) and improving the experience for end-users by avoiding unbearably small text. Qt developers have been working on HiDPI support for several months.
While there's currently outstanding patches for Qt HiDPI support, there are some issues still present and based upon the discussions, the work is on hold. Digia's Sorvig Morten wrote in the message that started the thread, "Based on the patch reviews on gerrit and some discussion on irc I’ve decided that we don’t have a good enough consensus to merge the patches. I’m putting them on hold until after the summer break at least."
Delaying the Qt HiDPI patches until after the summer puts it out of reach for Qt 5.4 and potentially Qt 5.5 depending upon how much further work is needed for tidying up the patches.