Checking the manual for the WD15 dock (link), the Display Resolution Table on page 31 gives different possible resolutions depending on the dock cable used.
It refers to the "Universal Cable". When used with a Thunderbolt laptop this should give 4k resolutions at 60Hz. The WD15 FAQ (here) lists the Universal Cable as being available summer/autumn 2016. I can't find it on the website, is it available yet?