They should be here Tuesday, though knowing Amazon, I expect them here on Monday.
From memory, the HE-4 has a slower, lingering bass than the HE400 (I actually want that), and similar treble. They're quite similar, though the HE400 is a little darker and more natural sounding. I'm more interested in the HE-4 for the bass, as the HE-400 didn't have enough for me, and I remember the HE-4 having somewhat similar bass to the DT990, though we'll soon see. I think the mids are a little more recessed than the HE400. So overall, the HE-400 is definitely more appealing to the head-fi crowd, while the HE-4 appeals more to V-shaped fans (which I happen to be). It was like a DT990 with less screech in the treble, and more warmth. So basically, the HE-4 is to the HE-400, as the DT990 is to the DT880.
In a perfect world, I'd own both.
For everyone else to see how the HE-4's innards look...