I remember reading in a review somebody had said that the 4ai is like the B2/DBA02/DBA02mkII/R-50/GR01/CK10 on steroids. Supposedly its somewhat similar to all the TWFK based dual armature IEM's but on steroids. The 4ai uses the TWFK for mids and highs and uses another different dual armature for bass. Same great mids, same great, but smoother highs, but with a better bass.
Honestly I have no desire to upgrade from my GR01, because for what I use it for, it is perfect. I wouldn't want any more or any less of anything. In my opinion it is absolutely perfect for rock, prog rock, and metal, and its even good for some of the electronic music. When I want more bass I just swap out for my dual dynamic driver Brookstones. For rock and metal the GR01 is just what i was looking for. The GR01 may actually be an upgrade from the B2, not a big one, but it's supposedly a slightly milder/warmer B2/DBA02. I've never heard the B2, so I cant say for sure.