My wife loves the X6 but its out of our price range, fully loaded X3 is our max for now. We will probably be looking to replace our X3 around the time this comes out and I am sure it will be considered, BUT there are two major concerns for a family SUV:
1) obviously the cargo space will be smaller than the X3 since its the same platform and size length BUT coupe like styling meaning trunk is probably cut in half (volume wise).
2) Price! the X6 is ridiculous, much higher in price than an X5. If the X4 is a few thousand more (like $3K) than the X3 (with similar options) I think it will sell well.
Now reasons I would still consider it despite these concerns:
-If they can keep the cargo space floor dimensions the same (or maybe a little wider) and if our double stroller (which fits in the X3) can fit in the X4, it will be on the list.
-$3K more for unique styling and hopefully a little better handling and MPG than the X3 would be fine with me, any more and I would just get another X3. Hopefully they will also add more luxury options not available in the X3, like Nappa/Novillo leather and multi contour seats, that alone would make a bunch of potential X3 owners jump to an X4.
For those who think people looking at an X4 will just get an X5 for the same (or close to it) price, that probably is not the case. My wife and I for instance do not want anything as large as the X5, price doesn't really have anything to do with it. We got our X3 for $55K MSRP, which could have easily bought a more basic X5, but it was too large and the MPG is much worse.