In practice, Tesla on HW4 drives indistinguishably different from Waymo.

It does! A system which drives indistinguishably different from Waymo 99.999% of the time is L2. You might very well never experience that unlucky 1 mile in 100,000, but if there's 1M Teslas on the road driving a daily average of 33 miles, it's going to happen hundreds of times each day. An L4 system must guarantee that it can come safely to a stop before human intervention is required, and I don't think you can achieve that guarantee by pushing the nines on an L2 system.