The XP has upgraded software which is supposed to make improvements to the battery life and heat up time. As well, the heating coil is slightly different; wheras the INH004 has a coil that is made of the usual style of wire, which is round all over. The XP uses wires that have a flat side, and that flat side rests against the inner lining of the heating chamber. This will speed up temperature control.
All in all, the changes are small. If somebody already owns the 04 I don't think that there is really that much to be improved on because the vape is so damn good already. It's a great iteration of the product though, and I like the look of it alot better than my older INH004.
And as for them mentioning another vape, your proabably thinking about the INH005 which they have mentioned a couple of times but it will not be ready for a while.