[NAME] managed to add the missing resolution 3840x2160 with the Custom Resolution Utility (CRU) v1.5.1 from: [URL] on my 9310 "Non-2-in-1". Don´t forget to reboot Windows after adding this custom resolution to get it working!
I can only select 4K 3840x2160 Resolution for internal display when I connect an external Monitor. Or connect a 4K Dummy Display Adapter to USB-C-Port. Without connecting external Monitor I only get Choice of 3840x2400 or 2560x2048 or lower resolution options: No 3840x2160 option. I tried under Win10 and Win11 21H2 completely up-to-date 2022-01-08. The same result. I use newest Intel Iris Xe Graaphics Driver 30.0.100.9955 (2021-10-07). I this something that has to be implemented into BIOS of "XPS 13 9310 2-in-1" or can it be corrected by the Internal monitor driver (DELL SHP14F8 Display, Version 1.0.0.0 from 2020-05-12)? I want to use my "XPS 13 9310 2-in-1" with 3840x2160 without the need to connect an external monitor or Dummy Display Adapter and I don´t want to change Resolution everytime when I connect external 4K 3840x2160 monitors.
XPS 13 9310 2-in-1 : Can only select 4K 3840x2160 Resolution for internal 
display when external Monitor is connected