That's the first I've heard of the HEX4, on their website they say it's a mix of the HE4XX and HE5XX, they've got a frequency response measurement done on a B&K that shows HE4XX vs HEX4:
On the plus side for the HEX4, it has better bass extension to 20Hz (but loses out in the rest of bass, so no real change in bass abilities when thinking about it), although it doesn't have the 5.5kHz dip, so it might turn out (when measured on a more bonafide GRAS unit) that HEX4 is easier to EQ to the Harman Curve than the HE4XX. Yeah, I'd like to see the HE-X4 reviewed.
EDIT: also less distortion in the HEX4:
EDIT#2: not sure why the HEX4 is only $129 and the HE4XX is $180, as the distortion measurements would indicate HEX4 is technically better, however it's probable that the HE4XX is a more accurate headphone without EQ, as the HEX4 is lacking even more energy in the 1-3kHz area, which is also the same area that the HE4XX needed boosting, so HE4XX is probably a more accurate headphone out of the box.