The product brief sheet for D4 just says that it has a 128 bit disparity search.
I could not find any data about whether the 400 Series' disparity was fixed, but its stereo camera predecessor the R200 had fixed disparity, apparently.
A blog article published a week ago by Sergey "Dorodnic" Dorodnicov, the RealSense SDK Manager, listed one of the challenges that stereoscopic vision products in general have to overcome as "expanding the range of generated disparities from fixed set of integers to achieve sub‑pixel accuracy".
After more searching I found another info about 128-disparities search (not 128 bit, obviously)
Concerning the "expanding the range of generated disparities from fixed set of integers to achieve sub‑pixel accuracy".
It's not connected with disparity range - it means expanding integer disparity representation to float or fixed-point to improve disparity accuracy.