I'm planning to buy 2 Yongnuo YN-560 IV flashes and some receivers/transmitters.

You need to decide which triggering system you want to use and then commit to dealing with its limitations. Here are your choices:

• Use optical triggering with the 430EX II-RT and the YN560-IV, in which case you only need an optical master unit to trigger everything.

• Use Yongnuo's radio triggers, and accept the limitations that has for controlling your 430EX II-RT.

• Use Canon's radio triggering system, and either choose a receiver for the YN560-IV that works with the RT system, or choose a different flash that works with the RT system.

What you can't do is mix Yongnuo's simple radio triggers with Canon's much more complete radio system. The reason the YN-E3-RT can't fire the YN-560 IV is that the former is a transmitter compatible with Canon's RT system, and the latter has a receiver that's part of Yongnuo's system.

If it were me, I'd pick Canon's RT system, even if I were choosing Yongnuo products. For example, you could buy two YN600EX-RT II and a YN-E3-RT, and then you'd have a system that doesn't need any separate triggers and which gives you full control over power settings for each unit. Or, if you really want to go with the YN560EX-IV, you could trigger that with a YN-E3-RX. But since the cost of those two together is nearly as much as that of a much more capable YN600EX-RT II, that doesn't seem like a great option.

If you choose Yongnuo's radio system, then you need to accept the fact that the RF-603 can't control power on the 430EX II-RT. Consider replacing the 430EX II-RT with a third YN560EX-IV, and throw in a YN560-TX, and you've got a three-flash system with radio triggering and control over power for each one.