The Blue interfaces will work with the HDX cards. I found this out on the chat that's posted here (on the right):
http://apps.avid.com/aes2011-live/
My Sweetwater rep said that for now they're only selling the bundles with the interface, but that may change down the road (don't know when).
Since you have an HD4, your trade up options would be this (I'm pretty sure):
You trade your HD3 system for 1 HDX card for $5999. Your blue interface would then be an even swap for the HD Omni. Or you trade your blue interface and add $1000, and you get the HD IO (8 x 8, I think).
Then, if I'm not mistaken, your extra 4th card could potentially be traded for a second HDX card, but I have no idea how much that would be for.
I'm just going by what was said in the chat, but you may want to talk to your dealer.
Note that these are the official street prices (MAP). Your dealer may be able to do even better.