Watling Towbars do a towbar for the LS400 - see www.watling-towbars.co.uk
Click on Price Lists on the narrow blue horizontal band, scroll down to Towbars Pricelist - Jan 2013 and click on this. Lexus are on p22. They are not cheap to buy - £264.00!!
I have one fitted to my 1997 LS400. I did not fit it or have it fitted, as it was on the car when I bought it (and it was one of the reasons I bought the car).
Mine is a Mk 3 car, although they will fit the Mk 4. You will see from the Pricelist that the towbar goes up to Jul 98 i.e. the end of R reg cars, so if yours is a R reg car that will be fine.
EU Type Approval for Towbars came in on 1 August 1998 and, although the towbar will fit S reg cars onwards, it is not a Type Approved towbar. As I understand it, it is a legal requirement for vehicles registered after 1 August 1998 to have only a Type Approved towbar fitted and it will be an offence under the Construction & Use Regulations if a non-approved towbar is fitted. It might also affect your insurance.
Hope this helps.
Colin