The mini display port supports a maximum resolution of 3840x2160 @ 60hz, 24bpp which is exactly four times HD and 4K terminology is used interchangeably to refer to that resolution. You may want to check the Technical Product Specification for your product for further information: https://www.intel.com/content/dam/support/us/en/documents/boardsandkits/NUC5i5MYBE_TechProdSpec.pdf
I would also recommend that you check the following website for further information on how to enable your system for 4k playback https://www.intel.com/content/www/us/en/support/articles/000029357/intel-nuc.html
In addition to the previous reference information, you may want to ensure that the system is running latest drivers and firmware, refer to the following website for the most recent software: https://downloadcenter.intel.com/product/84861/Intel-NUC-Kit-NUC5i5MYHE
Finally, keep in mind that interactive support for this product has been discontinued, I would encourage other community members to assist on this issue.