vlc 4k choppy while mplayer is smooth

Chances are you can offload the video playback onto the video card, instead of using the CPU(s).

Make sure you have the vaapi packages installed (for Intel/AMD video card):

sudo apt-get install libva1 libva-x11-1 vainfo vdpau-va-driver

If you have an Nvidia card add this:

sudo apt-get install vdpauinfo libvdpau

Since mplayer is working fine, it may already be using hardware acceleration. So these packages may already be installed.

Next, see if VLC will start with hardware acceleration:

vlc -v /path/to/name-of-file

There also is an option in the GUI to turn on hardware acceleration. Check to see what it is set on (Tools>Preferences>Input/Codec):

Is there a fix?

Answer to the question: it depends.

I think there are some other questions you have to ask in order to solve your problem of not being able to play 4K video without it being "choppy" using vlc:

• What videocard (GPU) do you have?
• Do you have the correct driver installed?
• are these drivers being used?
• Is mplayer using hardware acceleration?
• Is VLC using hardware acceleration?
  • if yes: why is it not playing smoothly?
  • what codec do you need?
  • etc.

Hint

From your findings I guess mplayer is using hardware acceleration, since I don't know a CPU that can playback 4K video, so I guess that you probably have a GPU that is good enough to playback 4K.