Both. For my mid-2014 15" MBP I had to get an active MiniDP-HDMI adapter. Looks like your MBP is limited to 30Hz...

https://support.apple.com/en-us/HT206587

Interesting! So it's not the monitor (TV), it's the MBP.

Oh well, will have to wait for the next MBP 15" refresh--don't see any real reason to upgrade yet to the latest TB MBP.

Plus, the @30 doesn't normally bother me, unless there's a solid large gray-ish color, which definitely shimmies.

These 40" Samsungs 4Ks really hit the sweet spot for Retina displays, at least for my old eyes. "Only" 1080P logically, but oh, so large and sharp!

Also, did you have to do any SwitchResX hackery? That's the only way I could get macOS to recognize the monitor as a Retina display.

Nope, but I run it at 100% scaling. I did notice that if the source on the TV is not labeled "PC" then the Mac uses YCbCr 4:2:0 and looks like garbage. Setting the input label to PC must send a different EDID because it seems to use RGB... On my Windows machine I can just override that in the nvidia control panel.

Oh, you run it at the full 4K (3840x2160) resolution? Must have good eyes.

Right, nothing works unless you label the relevant HDMI input "PC".