@Dave_ZPCMR I plugged in a 1920x1200 monitor (yes a slightly strange size) and could reproduce what you saw; much higher fps on single rider views on the 4K game resolution setting. So the presence of the real 4K TV is limiting the frame rate in the 4K setting compared to a lower resolution monitor.
I now understand why you thought I should be able to get 60fps easily (I can’t)!
And the suggestion to stick with 1440p is a good one. I can’t particularly see any worsening of the graphics, and this is confirmed by another post here that frame rate might be preferable over resolution.
Having a decent frame rate and the ultra profile activated makes the biggest difference to the experience, the resolution less so (at least between 1440 and 2160).