Hi,
the latest versions of VLC are able to play with no issues 4k contents, you can get the nightly builds directly from here: http://nightlies.videolan.org/build/android-armv7/
Otherwise the new version MX Player Pro v1.7.31 supports now the K1 natively but it crash often, I tested it.
As far as I see some video are played directly smoothly with the built-in player.