I think I'm going to jump in the boat with Dewfang on this topic. I think we are making too much about this and how the T4R bolt on hitch is inferior to the Lexus one.
But my position on this just changed last night when I was looking at the hitch on my 2004 Dodge 3/4 ton 4x4 Cummins desiel which is rated at something like 1,000lbs tongue and 12,000 capacity using weight distribution hitch. Because the hitch is basically the same as the one on the T4R, as it bolts on to the tubular cross member that mounts to the frame.
Here is a picture of the Dodge bolt on hitch and as you can see, the majority of the weight is secured by the vertical 2 bolts and the T4R's bolts aren't vertical so they aren't relying on the threads but on the thickness of the horizontal bolts. Granted the bolts on the Dodge are larger but we are talking about over 2 times the towing capacity.
So I think the stock hitch is just fine for the rated capacity, 5,000lbs, and it's fine to use a WDH like Dewfang mentioned.