I don't think an X4 would have any more "volume" sales than the X6 does. Just like the X3 and X5 are the more conservative styling (and pricing) for the masses. The X4 can fill a niche, but that's about it. I bet it will be cool.
2016 M3 - on order for October 19th European Delivery
2013 ActiveHybrid 3 - sold
2012 M3 coupe - sold