YN560 iv And Canon 70D

So now I have 3 unit of YN560 iv and 1 unit of YN560-TX radio trigger. Here I'll will brief how I use the flash in 3 ways, on hotshoe, optical wireless and YN560-TX as a wireless master control.

1. On camera Hotshoe

Canon 70D's menu cannot recognize the flash, but it will still work. If you put the YN-560iv on the flash hotshoe and push the shutter button, the flash will still fire in sync with the camera shutter. But you have to set/adjust the output power manually on the back of the flash. If it's not firing, check the batteries, and that the flash is seated correctly on the camera hotshoe.
Also you have to make sure that the flash is out of any slave modes, though, when you mount it onto the camera. S1/S2 tells the flash to stop "listening" to its foot and to listen to the sensor panel instead.
Obviously, you won't have menu control, TTL, HSS, etc. etc. since, like the YN-560III, the IV is also a manual-only flash and only has the sync pin.

2. Optical Wireless

 This is an optical trigger and requires line-of-sight to the off-camera flash. The built-in flash uses pulses of light (visible light) to communicate with the slaves and send instructions.  It's not infra-red nor radio.  But receiver in the slave flashes is on the lower body (not up in the flash head).  The front of the lower body has a red-tinted panel which resemlbes what you'd see on an IR remote control -- leading people to presume that it's IR.  It is actually visible light -- not IR.

The YN560 iv can do slave flash with your camera, but it is a very basic form of slave flash where it will respond when it sees a bright flash of light from your camera's flash or any other flash it sees.

In order to use the YN560 iv with your Canon camera you must set the camera's on-board flash to normal ETTL flash or normal Manual flash. (Make sure your camera flash is not set to wireless because the addition optical wireless "Flash Commands" will just confuse the YN-560 and it will not sync properly)

The YN-560 does have two settings for optical slave flash:

"S1" is used when you have your camera's flash set to manual mode. The YN-560 will then sync at the speed of light to match the flash from your camera.

"S2" is used if you have your camera's flash set to ETTL. The YN-560 will ignore the first ETTL preflash and then fire when it sees the main flash from your camera.

To avoid seeing the flash from your camera in your photos, adjust your camera's flash to a low power setting or use a piece of white cardboard to deflect to camera flash away from your subject.

In my case I got better result using "S2" slave mode.

3. Radio Trigger - Using YN560-TX Radio Transmitter

This is the best way. Ideally, what you may want to consider, is getting a YN-560-TX transmitter for the 70D's hotshoe (instead of using the pop-up flash), which then frees you from the optical "line of sight" and range restrictions by using radio instead to trigger the off-camera flash.

Set 560-TX to TX mode (TX means it is set to master and ready to trigger a slave)
Set 560 iv to RT mode (RX means that the radio is active and now the unit is ready to be triggered by a master)

Set the same channel for 560-TX and all 560 iv speedlights.
Assign a group to each of 560iv speedlights.Group 'A' ,'B' and 'C' (in case of 3 units of 560 iv)

The 560-TX can control up to 6 speedlight groups (A,B,C,D,E,F),utilizing 16 channels at 2.4 GHz frequency with a transmission range of up to 328'.The YN560-TX will support all the major functions of the remote YN-560 iv speedlights.

The YN560-TX is a remote speedlight controller ONLY (radio transmitter only). The YN-560 IV is a remote speedlight controller AND a speedlight with built in radio transmitter & receiver.. That means that the YN560 IV not only can act as an on-camera speedlight, but it can also control speedlightes that are not mounted on the camera. If you are not interested in having an on-camera flash, then this YN560-TX speedlight controller will work to control off-camera speedlightes, but it mounts to the camera's hot shoe and you can't mount another speedlight on top of it.Some people consider redundant to have both 560-TX and 560iv.
I wouldn't consider it redundant to have both, 560-TX and 560IV. I want my trigger ON my camera and to have my speedlights OFF the camera.