I guess I was wrong about all ATF-4 being synthetic in my earlier post. The ATF-4 stuff is just a formulation spec. As near as I could determine it does start with a higher quality base though. I searched and found a technical abstract on the ATF-4 but it's way over my head.
http://u225.torque.net/cars/tech/trans/982674.pdf
However, here are the conclusions from the article which do make some sense.
CONCLUSIONS
This paper describes the development of an ATF that is
considered fill-for-life. The bench and field test data
presented above supports the following conclusions:
1. The Group III XHVI base stock used in ATF B
provides superior low temperature performance
when compared to other ATFs in the same category.
2. ATF B has excellent shear stability and viscosity
retention as defined by the 20 hour KRL test and
shown in the vehicle data.
3. The oxidation stability of ATF B has shown significant
long term improvement over other fluids as shown in
the ABOT test and vehicle data.
4. The anti-wear properties of ATF B have been
improved measurably over ATF A as defined by the
Modified ASTM D2882 and D5182 tests and shown
in the vehicle data.
5. The frictional properties of ATF B closely match
those of ATF A and are substantiated by various
bench and vehicle tests.
Maybe some of the more knowledgeable transmission guys can step up here.