The 4WS system is a fairly complex setup (also it is a Jap option).

There are five main components, main motor, backup motor, speed sensor, steering sensor and 4WS computer so it could be any of these, also add a possible wiring fault on top of that.

You can carry out a diagnostic test to interrogate the 4ws management fault code by bridging Tc and E1 connections at the diagnostic plug (situated next to the top of the passenger front suspension strut in the engine bay). With these two connections shorted you should get a series of flashes from the 4WS light on the dash when you switch on the ignition. Each series of flashes represents a digit which can be looked up.

Hi Mike,
I am assuming you replaced the 4WS sensor which is located next to the two 4WS motors.
If you trace the wiring of this sensor it will lead you to a connector in the boot. Remove the panel cover and you will see it.
This is a 3 pin connector. Disconnect it. Take an Ohmmeter and measure the resistance between Pin 1 and Pin 3. Divide this by 2 and 
this is approximately the value you should see between Pin1 and Pin2, and also between Pin3 and Pin2.
If they are unequal, you will have to loosen the two screws on the sensor. These are mounted in slots, so that you can adjust the sensor 
by turning the sensor. Tighten back once you can get the two values to be approximately the same. Hope this solves your problem. Good luck.
Regards,
Robert

Hello Robert,
Many thanks for your advice on the matter.
As the mechanic at the Toyota main dealer fitted the sensor, he said it was the first time he had ever worked on a 4WS. He explained that he 
fitted the sensor the same way as he removed the old one, presumably meaning just by its "visual orientation".
How would I adjust the sensor for proper balance? Would this adjustment bring the 4WS back to normal operation even though the diagnostic
code is saying 32 = Back Up Motor/Steering Control Relays? (Steering control relays (engine bay) have been checked & are OK).
Regards
Mike Bishop

My 4WS started to intermittently switch on and off (warning light on dash) when I came to drive it. That is to say, some days it would work
then other days it wouldn't. Now it's got to being permanently off 4WS & the dash lamp stays on all the time (apart from the initial 10 secs
delay/30mph, once driving). So far I've diagnosed the codes to 2 faults. steering sensor on the rear assembly & the "back up" motor.
I've just had the sensor renewed by Toyota Main Dealer (part came all the way from Japan, I think I paid for a guy to hand carry it here
judging by the price!). This has not cured the 4WS problem as dash light still comes on after the initial delay, but the fault codes now
only show 1 code.....the back up motor. Also the 2WS/4WS dash switch does nothing, which I believe is what the backup motor is 
operated by??
Any ideas which motor on the rear assembly is the backup motor? any advice on where to hit it with a hammer? It's probably got brushes
wear, I'm guessing, same thing as the motors in the retractable wing mirrors.....but I sorted that one.
Mike