Does the 85mm f/1.8mm Produce that much shallower DOF than the 70-200 f/4 ?

Another thing you can look at is an online DOF calculator, e.g.

http://www.dofmaster.com/dofjs.html

Plug in (say) full frame, 2 meter subject distance:
DOF @f4 =   29cm
DOF @1.8 = 13cm

DOF is very sensitive to subject distance. The sample pictures above may help more than the calculator (although the image at 1.8 looks to be front-focused to me though...)

-V