I've been running 16.04 beta for the past month on a Thinkpad P70 with a 4k display, and scaling has been great. The OS UI elements (like menu bars) can be scaled to any size.

Most applications seem to work out of the box. The ones that have given me the most problems are a handful of desktop apps implemented in an embedded browser (like Spotify) where I have to set an extra command flag to properly scale.