Regarding the lack of a 4K option in Mac OS via USB-C or DisplayPort, the reason may be that the Mac needs 4-lane bandwidth Main Link to reach 4K resolution on external monitors. Otherwise, a no 4K option issue may occur if DisplayPort/USB-C provide only 2-lane bandwidth. Thus, we suggest you follow these steps as a workaround.
●For SW271C/SW321C/SW271:
Go to OSD menu > System > USB-C Configuration > UHD@60Hz + USB 2.0
●For PD2705U/PD3205U:
Go to OSD menu > System > USB-C Configuration > USB 2.0