Yeah I have one in my Hamer Mirage.
It's not a million miles away from the JB and maybe closer still to the JB2, but I seem to like something about the A4's frequency response, so for me it's an improvement.
What I hear is a more balanced mid range that seems to be smoother over a wider range rather than the more extreme emphasis that the A5 has. It probably has more mids but as it's more even it's not quite as harsh let's say as with A5, and not quite as... what's the word, squishy?...as A2.
I'd like to try an uoa5, but of A2,A3,A4, A5 & A8 that I have done, it's my clear favourite.