I would like you to check the different resolutions supported while using a single or multiple displays with the NUC6i5SYK.
This information would be found in the page 22/23 of the following PDF: http://downloadmirror.intel.com/25507/eng/NUC6i5SYB_NUC6i3SYB_TechProdSpec01.pdf
4096 x 2304 @ 24Hz would be supported via HDMI
3840 x 2160 @ 60 Hz via DisplayPort
If you require any additional information, feel free to contact the Intel Communities again.