2018 MacBook Pro + Dell U2718Q, single cable for display and USB data Can I connect a 2018 MacBook Pro to a Dell UltraSharp 27 4K Monitor (U2718Q) with a single cable that carries both the display data and the data for the USB ports of the display?

I tried a USB-C to DisplayPort cable, but none of the USB ports of the display would work until I also connected it to the MacBook Pro with a USB cable through one of it's USB upstream ports.

Note that I want to operate the display at full resolution (4K, 3840 x 2160) and a refresh rate of 60 Hz.
No.

Technically speaking, you have a Thunderbolt 3 Type C port.  "C" is the physical charastic of the port, not the USB specification.  Being that it's Thunderbolt 3, its carrying several signals:


PCIe
USB 3.1
DisplayPort
Power


Your "end" device is a monitor that has several different inputs


HDMI
DispayPort (DP)
Mini DisplayPort (mdP)
USB


When you go from your MacBook's TB 3 port to the video in, you're not passing the USB signal; this is why you need a second cable for the USB functionality.  If this was a monitor that accepted Thunderbolt as an input, you could do this with one cable.