We have two Samsung 4k monitors (U28E590) for editing and a Vizio 4k 50″ TV (E50u-D2) for client review. This is running off a 2013 Mac Pro running OS X 10.11.6 and FCPX 10.2.3. The two monitors are Thunderbolt to DisplayPort and the TV is HDMI. This enables us to run 3x 4k displays off the Mac Pro.
When setting up A/V Output in FCPX I found that the item was grayed out in the menu unless the TV was set to 3840×2160 or 1920×1080 in OS X Display Preferences. Once I set the TV to 1080 or 4k, A/V Output was available in the FCPX Window drop down.
The issue we found is that the playback on the TV lags behind the audio of the computer. Not much but definitely enough to bother the client. If you send the audio to the TV as well it is still out of sync. Apple states, “Video and audio are synced at the video frame (not audio sample) level” so I assume sync is pretty much dependent on how fast your TV is as there is not offset setting in OS X.
Hope this helps those that cannot get their TV to show up under A/V Output.
P.S. if any of you have a fix for the audio sync it would be fantastic!
Video Editor & Producer
FCPX / Adobe Premiere Pro
http://www.natemccallister.com