There have been rumours about a B&M 4D coaster since the beginning of time (well, since X was built anyway). I've never seen any compelling evidence to suggest there's one in the works. I think it's just wishful thinking on the part of enthusiasts.
Personally, I'm not too bothered about the idea of more 4D coaster models. From the late 80s through into the early 2000s, a lot of coaster innovation seemed to concern "what kind of crazy seating arrangement can we put on a track?" While this produced some successes such as the inverted coaster, it also produced a lot of gimmicks that either add very little or in some cases detract from the ride.
For me, the Arrow style 4D coaster is the most extreme example of train-based gimmickry. It's a fun concept and it has great novelty appeal but in some ways it's just introducing technology for technology's sake. I wouldn't want too many to be built in place of potentially better conventional coasters.
I prefer the direction that the coaster industry is currently going: ie. Focusing on building coasters with really good, high quality layouts, devising outlandish elements and pushing track technology.