Yongnuo claims that the shutter release and the flash transmitter are two separate functions and that you can use just one RF-603 in the camera hotshoe to do both functions. They claim that you can now do what the OP wants to do with only 3 units. I have never seen any real world reports if this works or not. This is their explanation to why the button on the RF-603 is only for remote shutter release and cannot be used as a flash test button. They claim that when that button is pressed on the handheld unit, the hotshoe unit will trigger the shutter via the shutter cable. When the shutter fires the other half of the hotshoe mounted unit will send a fire signal to the remote unit connected to the flash and fire that. It would be great if someone with the RF-603's could confirm if this setup works.