The YN560-IV is a manual only flash. It does not do iTTL at all. It does have a radio receiver built in that allows you to control the flash's manual power and zoom head, as well as fire the flash, from the YN560-TX transmitter. You can also tell it to "fire" using an RF-603 N/RF-603 II N/RF-605 set to "transmitter" on the camera, but you can't set power levels and the zoom head remotely with them.

The YN-568EX N is a full featured iTTL flash, but it has no built in radio receiver. To use it off camera via radio you need a transmitter/receiver set. It can also be controlled optically via the Nikon optical wireless control system.

If you use a YN560-TX on the camera and an RF-603 N/RF-603 II N/RF-605 attached to the flash as a receiver you will only be able to use the YN-568EX N flash in manual mode. The YN622N/YN622N II transmitter/receiver system (each unit can act as either a radio transmitter or radio receiver) will allow you to use the YN-568EX II N off camera with the full TTL capability as well as HSS in exactly the same way you can use the flash when mounted directly to the camera's hot shoe. There is also a YN622N-TX transmitter that has an LCD screen that makes it much easier to see what you are doing, especially when controlling multiple flashes/groups of flashes. Unfortunately, the YN622 radio system can not control the YN560/RF603/RF-605 radio receivers. A YN-560TX can communicate with the most recent version of the YN622N-TX II and allow it to pass through commands to YN622N II compatible receivers and flashes, but the reverse is not the case - A YN622N II can't use the YN560-TX to pass commands to flashes/receivers that use the YN560/RF-603/RF-605 system.

The YN685 N is also a full featured TTL flash similar to the YN-568EX N, but it has a built in radio receiver that can be controlled by either the YN622N system (with full functionality, just as if the flash were directly mounted to the camera's hot shoe) or by the YN560/RF603/RF605 system (with the limited functionality of that system).