You are right the LS400 4 piston rotors and calipers do fit on to the front of the Aristo but there is some work required.
You either install roll centre adjusters as stated or a lot of people actually grind off some of the caliper material and outer tie rod end to make them fit as the calipers will not bolt on as they hit the outer tie rod end and hub.
I personally would never install a caliper on my own vehicle which has had material removed right near the brake fluid passage as have driven a car before when the brakes failed and its not a fun experience!
I have my new pads and rotors here waiting to go on my car but i am waiting for my stainless hoses to come before i do the job.
As soon as there on i will post up my findings.