If you want to use the radio master/slave setup, the 1D4 will sync at 1-stop less than your normal max sync speed. So you usually sync at 1/320 sec –*with radio mode, you will be at 1/160 sec.
I just tried my 680EX-RT's with my 7D and found a few things:
1. Group mode does not work,
2. A lot of functionality is lost with the Radio triggering. I wasn't even able to control groups A and B power from the slave flashes.
3. Infrared triggering works just as before.
4. I saw no exposure difference between my typical max sync speed of 1/250 sec (7D) and the recommended 1/125 sec with off-camera flash.
If you're looking to invest into the 680EX-RT system for your 1D4 for the radio system, I would probably hold off. If you see a 5D3 or 1D4 in your future, it's a great system.
I spoke too soon, the 600EX-RT's will do the same Manual and E-TTL ratio control for Groups in Radio triggering mode. In Manual power settings, I was able to control Groups A, B, and C individually. In E-TTL mode, you can control Groups A:B, A:B + C. Even at my normal max sync speed of 1/250 sec, I noticed no difference in exposure.
With that said, I would definitely make the move to the 600EX-RT system on your 1D4. The off-camera power control is not as intuitive as on a 5D3 or 1Dx, but it still works well from my experience.