[brochild]

 <rules action="prepend">
    <rule filename=".*4k.*|.*UHD.*|.*uhd.*|.*uhd.*" player="MPC-BE"/>
    <rule filename=".*.3D.*|.*.3D.iso|3D.*" player="MPC-BE"/>
    <rule filetypes="bdmv|mpls|m2ts" player="VideoPlayer"/>
    <rule filetypes="3d" player="MPC-BE"/>
</rules>
</playercorefactory>

 Twinpeaks:
I had the same issue playing my 4K movies using external player MPC-HC.
Try the following:

 <rules action="prepend">
    <rule filetypes="bdmv|mpls|m2ts|iso" filename=".*4k.*|.*UHD.*|.*uhd.*|.*3D.*" player="MPC-BE"> </rule>  
 </rules>


Here's a copy of my file to play 4K movies. They all have either 4K or 2160 in the folder name and in the movie file name.
<playercorefactory>
  <players>
    <player name="MPC-HC" type="ExternalPlayer" audio="false" video="true">
       <filename>C:\Program Files\MPC-HC\mpc-hc64.exe</filename>
       <args>"{1}" /fullscreen /close</args>
       <hidexbmc>false</hidexbmc>
       <hideconsole>false</hideconsole>
       <warpcursor>none</warpcursor>
       <!-- ELJ set watched movies timer for 30 minutes -->
       <playcountminimumtime>1800</playcountminimumtime>
    </player>
  </players>
  <rules action="prepend">
    <!-- ELJ add for UHD MKV files, UHD ISO images, UHD Blueray folders -->
    <rule filename=".*4K.*|.*2160.*" filetypes="mkv|iso|bdmv"  player="MPC-HC">    </rule>    
  </rules>
</playercorefactory>