As far as I can tell, it boots but I can almost never see anything on my LG. I had that one stretch of about 30 minutes where it was fine. And then when I clicked the display settings to default, after the screen blanked and came back on, it was no longer connected.
Besides that one time, I have had 3, maybe 4 times where I would boot it up and that process on screen for 1 to maybe 10 seconds (usually 1-2).
On the other TV, I have gone through multiple attempts without success and then suddenly get it working through repeated cable changes - and then have a failure.
I have probably, at one point or another, used 4-5 HDMI cables.
I could be suspicious of the cables for 4k, but then why does the 1080p TV often not work either? It just seems there is no logical pattern.
BTW, when the iMac was connected, it still shows up fine as long as the TV is turned on before waking the Mac. Haven't worked out the time gap required. A few seconds wasn't enough. I think 10 seconds worked.