The problem with the 4x4 is not the brackets for the calipers, it's the disc offset. Both the 300 mm ST170 discs and the 320 mm FRS discs have too much offset which means the disc will hit the steering ball joint.

Perhaps the E38 calipers can be used with a set of 320/330 mm AP/HiSpec/etc. discs that have the correct offset for the 4x4. You only need to check that the disc thickness matches the original thickness of the E38, otherwise the disc might be too thick.