To defend my B&M 4D prototype statement a little, I'll say this:
As soon as I mentioned a 4D coaster, it seems as if everyone began comparing to the other 4D coasters. This is understandable, but impractical.
X2 has many issues because, yes, it was a prototype, but nothing even remotely similar had been done before its debut. B&M's 4D ride (if it happens) will essentially be a Revision A of their Wingrider model. I'm sure if they are working on the rotational seat concept, they aren't going to make it "unreliable" - years of testing and research has probably gone into this. C'mon, they're B&M and we know that they like to play it safe to a degree. They are known for reliability and I'm sure they want to make sure their product remains that way.
Secondly, Arrow coasters were notoriously rough. X2 was rough, which makes sense to me seeing as other rides made by this manufacturer can be painful. This doesn't necessarily mean that all 4D coasters ever made will be rough, though. Since B&M coasters are notoriously smooth, why would their version of a 4D coaster be painful and jarring? You could argue that the motion of the seats themselves may cause discomfort. An Intamin Zacspin (which swings freely) will experience much more rotational acceleration than a 4D coaster, therefore causing a much more forceful and out of control experience for the rider. The 4D coasters run on a "3rd rail" that can be programmed to rotate at any given speed/acceleration that the manufacturer chooses. Like I previously stated, since B&M likes to play it safe to a degree, I highly doubt they would put any intense rotational motion into the seats themselves - this would cause more discomfort to riders, and more maintenance for the park.
So, all that being said, I still think it kind of makes sense that this could be the first B&M 4D coaster. It could also be a ton of other things; heck I don't even really want my statement to be true, it's just my guess!