The only way to use a 750D to trigger an off-camera YN-560 IV without adding any other hardware is to use the pop-up flash to trigger the YN-560 IV in "dumb" slave mode.

The YN-560 IV has a built in radio receiver compatible with the YN560/RF605/RF603 communication protocol. Your camera has no built-in radio transmitter (none of Canon's current cameras do), much less one compatible with YN560/RF/605/RF603.

Your camera's built-in pop-up flash can communicate wirelessly with off-camera flashes that are capable of receiving optical wireless signals using the Canon optical TTL protocol. The YN-560 IV has no such 'TTL' receiver, optical or radio.

The YN-560 IV does have the ability to "fire" when it detects a bright flash of light. 'S1' mode is the basic optical slave that fires as soon as it senses another flash. 'S2' mode is the optical slave mode that (hopefully) ignores the TTL pre-flash and fires on the second main flash. Both of these modes will require your camera's pop-up flash to fire while the camera's shutter is open. Use straight 'manual flash' for the camera's pop-up flash for S1 mode. If you want to use the pop-up flash in TTL mode, you can try S2 mode, but that tends to be more problematic than S1 mode.

If you want to use the pop-up flash to fire a communication signal to an off-camera flash before the shutter opens but not fire a flash while the shutter is open, you should have considered the YN568EX II or similar flash that is optical TTL capable. The YN685 adds a built-in Yongnuo 622 radio receiver that allows full TTL control via a YN622 transmitter but removes the optical TTL receiver. The YN622 protocol is different from the YN560/RF605/RF603 protocol. Some YN622 devices made since 2014 can be set to receive YN560/RF/605/RF603 commands in manual only mode, but YN622 devices can not control YN560/RF605/RF603 devices.