The 4Ai isn't nearly as bright as I expected it to be, though it's treble response is pretty impressive.
Compared to the GR07, I think the 4ai's treble is better because it has more room to work with. It doesn't seem as compressed/congested as the treble of the GR07 does.
I've yet to hear any real sibilance with the 4ai, though I barely hear it with my gr07 either.
Is it just me, or do universals shaped like customs have better sound stage height? I noticed the same thing with my ASG-1, and now the 4ai. I just find that interesting.