2017 Macbook Pro + TCL 4K Display = No 60Hz
I don't know if this is a general answer or not, but this inexplicably worked for me....

The system seems very particular about what order things are plugged in. Here are the four ports of my laptop:

Back left &lt;--> Belkin Thunderbolt 3 Dock &lt;--> 5Bay TB2 HDD enclosure &lt;--> Mini-Displayport to Dual-Link Display Adapter &lt;--> 30" Cinema Display

Front left &lt;--> HDMI cable to TV

Both right ports &lt;--> None

If I plug the HDMI cable in first, it will connect/load and start displaying, sometimes at 30Hz and sometimes at 60Hz.

After it's displayed, I connect the dock and, during all the commotion of setting up the additional screen, it sometimes (or usually?) recognizes that 4K@60Hz HDMI is an option an uses it.

My best guess as to why? There may be a timing issue with the display discovery and this 4K display where the computer doesn't always think it has 60Hz available, but then, during the commotion, since the TV is already listening, the timing is better. But that's just a guess.