open a terminal and enter the code below:

sudo vi /boot/config.txt

Enter below lines to the end of the file:

# USB Type-C Host

Click ctrl+O to save the file and ctrl+X to exit the terminal. Reboot your Raspbian .

Disconnect Y-Splitter cable and connect any USB-C to USB-C cable to the middle USB-C port on NexDock. 

Restart your Raspberry Pi 4 and start using it as a laptop with only 2 cables.