To me the B400 is probably on par with the A4 on technical capabilities. That being said they're really different IEM's in many ways. The A4 is more lively, slightly V-shaped and entertaining to my ears while the B400 are more balanced, maybe even slightly mid forward. The A4 has a wider soundstage and better extension in both ends (and more bass slam) while the B400 is more intimate with great layering and separation. Although the B400 doesn't have the same subbass impact as the A4 they don't feel thin in any way and I'd even say that their midrange is thicker than on the A4.
The B400 does also have better ergonomics (although for me the B400 is also really good in this area) and a good deal better isolation. Using more balanced armature only IEM's has also made me appreciate how easy it is just to show them in and get great sound without having to worry about driver flex or what not.
Both of these are excellent in their own ways but most of the time I find myself enjoying the A4 more (personal preference) while the B400 is excellent for when I want isolation while still remaining a great sound. I use them a lot at work and they're great for me in that setting with their easy going and yet detailed, high quality sound. I bought them to use them in situations where I need isolation and don't feel to drag the Big Dipper with me and they're pretty much perfect for me in that role.
I hope this helps to give you an idea about how they compare.