Yes, you could get a 580EX (or 550EX or ST-E2) to be an on-camera 'Master' that will wirelessly control your 420EX as a 'Slave'.
You could also get a longer off-shoe cord. I don't think Canon makes them, but they are available.
This Canon wireless system does work, but it's not perfect. When using outdoors, for example, you will probably need line of site between units and bright sunlight can reduce the reliability.
Many of use use a radio trigger system. It's a more reliable connection between sender and receiver but with most units, you loose all the auto functions like E-TTL metering. What that means is that you would have to figure out the exposure on your own, perhaps with a flash meter.
One problem with that, is that (I think) the 420EX does not have manual controls. Many flashes have a manual power scale that will allow you to dial in the power it will fire at. Without that, your 420EX just fires at full power when not on the camera.
Hold on. You are saying that you have a 4
20EX, but your sig says that you have a 4
30EX. The 430EX does have manual controls.
One solution would just be to get a cheap flash to use as a remote off-camera-flash.
You could use a cheap radio trigger, or spend a lot more money and get a quality one that works reliably.
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