The reason the 3dr knuckles give a better roll center than the 2wd Sapph knuckles is because the connection to the TCA ball joint is in a lower position. If you compare the 3dr knuckles to Sapph 4x4, there is not that much difference in the distance from the wheel bearing to the bottom ball joint. The reason that the roll center on the 4x4 is not that good as on the 3dr is because the 4x4 front crossmember also has the connection for the TCA at a lower point than the 2wd crossmember. It has to be like that on the 4x4 crossmember to make room for the driveshafts. So if you would combine the 3dr knuckles with the 4x4 crossmember you will not get the same roll center as with the 3dr knuckles and a 2wd crossmember.

But the roll center is not the only thing to keep in mind for the front suspension. The caster setting, scrub radius, Ackermann and bump steer are also factors to take into account. Just trying to improve the roll center without thinking about the rest will probably make the handling worse.