GTX 660 + Dell UP2414Q 4K monitor=> Still no luck for me. The monitor is not recognized in MST (DP1.2) mode, so no 60Hz in 1920x1080 HiDPI, alas.
Try forcing HiDPI mode via terminal:
Code:sudo defaults write /Library/Preferences/com.apple.windowserver DisplayResolutionEnabled -bool YES
Then log in and out.