You might have to drop the refresh rate to 30 Hz in order to allow the 4K resolution to appear. 2560x1440 would be the maximum resolution you can run at 60 Hz on that display through the WD15, so that might be the issue. You could also try creating a custom resolution in the Intel Control Panel. Also note that you would not be able to run any other display through the WD15 when running a 4K 30 Hz display. If the single monitor and/or 30 Hz limitations ever become an issue, since you have an XPS 13 9360 you might want to look at the TB16 dock instead. It supports dual displays up to 4K 60 Hz and even some triple display configurations because it has access to 4x more video bandwidth than the WD15 since it uses Thunderbolt 3 rather than USB-C -- and the XPS 13 9360 includes Thunderbolt 3 as standard.