The Wiki covers it briefly here:
https://wiki.jriver.com/index.php/Video_Playback_Options#Display_SettingsYou will need to be connected to your JVC x500 projector in order to set it up for 4K @ 24fps. It looks like you are connected to your NEC PA272W monitor in that image, so only the resolutions and refresh rates it supports are displayed. MC queries the attached device to find out its capabilities, and displays those as options for Display Settings.
For example, I'm at my Workstation using a Dell 3008WFP monitor at the moment and it isn't any better than what your image displays. It only offers 50, 59, 60, and 75 Hz, which is consistent with the refresh rates available on my monitor for the various resolutions.
In either case, monitor or projector, try setting it to "On" first, and see if that sets your display output to the best frequency.
Otherwise, use "Custom" and select the best option manually.
As tij pointed out, your JVC x500 accepts a broader range of frequencies than your NEC PA272W.
JVC x500
Video input signals 480p/576p,720p 60/50,1080i 60/50,1080p 60/50/24,
3840 x 2160p 60/50/30/25/24, 4096 x 2160p 24
I see tij just beat me. I'll post the above anyway.