The Raspberry Pi 4 offers OTG functionality using the USB-C port which is normally used for power only. Therefore, for the PIKVM a special Y-cable is required and can be soldered as can be seen in this tutorial:
Is this way safe and correct? Since VBUS is disconnected, no reverse current can flow either from the host back to the Raspberry or from the power supply to the host. Only the Raspberry is powered, so I would assume this is safe.
I wonder that because I have also seen this splitter which makes use of Pu/Pd resistors, but I don't understand why.