I dont think a single monitor supports CEC, Also while you are right in what CEC does its really a standard for using remotes for one device on more than one device, so still requires a remote, or the device in question to be manually switched on in order to trigger the input change. This is not something a program would be able to do, at least not easily, to do what this user is asking there would need to be some sort of data transfer from the PC to turn on the device in question, and that cant be done over HDMI, as that is a display/Audio protocol, not a data one that can send/receive generic data from something like a PC. As far as i know there is zero way to turn on the PS4 from a windows machine, and therefore triggering the CEC on the TV to switch to the now active input once activated.
CEC is a useful feature in some situations, but i dont believe it applies here as the PS4 will still need to be manually turned on first, and what the user wants is for Launchbox to automatically do this when starting a Game in Launchbox, and thats without taking into account how you would even add these games and system to your PC in the first place, when there is no emulator to attach to this to even trigger the game starting and therefore powering on the system and making the input changes required.
It may be technically possible with a plugin if the power on over network that the PS4 has is readily available, but i dont know if that is closed source or not.