First off, the fit is slightly different- the C4 fits almost flush to the ear, especially at the bottom. The 4.A sticks out more overall, but the C4 has an extra "knob" at the top which helps with sliding in and out. This makes the C4 easier to slip in and makes it feel much lighter in the ear in my opinion.
Soundstage and imaging are simply untouchable on the C4, as mentioned previously, and bass slam is much more satisfying.
That's pretty much what I've noticed so far.
I may have missed it, but has anyone yet compared the C4 with UM's Miracle?