what i tell myself is that it's because ipv6 uses 128 bits, vs ipv4 32 bit addresses. so its 4x V4 addresses. Still makes little sense, but at least it's something, heh.
Christian Huitema is listed as an author on both AAAA and A6 RFCs, so maybe they know why the AAAA identifier was originally chosen instead of A6. Unfortunately, Google doesn't provide any "AAAA" hits for their blog at https://huitema.wordpress.com/.