I have both the X4's and now the X41's. The greatest difference is that now you dont have to be in a direct line of the receiver. It used to be when I would walk 10 feet to the left or right of the receiver it would static up and eventually just cut off, now i'm able to walk all around the house and even upstairs and still get crisp, clean audio (no more hiss!).
Another thing i've noticed is that the sound with the X4's sounded more like it was right in your ear, but the 41's have a bit of a "distance", so that it sounds more like everything is around you rather than being right up in your ear.
My only real complaint is that they made the earcups even smaller, but still big enough to envelop my ears and crush them so that they are pretty painful after more than an hour of use. Also, for some reason when I was playing my DS while using the headphones, the audio would keep popping and cracking up. I've had no problems with this issue when I wasn't playing the DS. No clue why its effecting it.
I recommend them though. They were $171 at Amazon after the eca code. I figure if I can sell the X4's for $100, a $70 upgrade to 7.1 isnt that bad of a deal.