> It's still domain dependent, right? If the rules say we're flattening precipitation and visibility within normal bounds, fine, but sometimes you have abnormal weather and badly-maintained roads

No, it's not.

In Level 4, if there's "abnormal weather and badly-maintained roads", the car must be able to deal with the situation and enter a safe state. It can say "please sit down and wait for assistance" but it may not say, "take over the wheel".

In practice you'd probably disengage the autonomy and drive yourself, you take control and it does not give you control. If you can't sit on the back seat drinking beer or send the car to drive your kids to school (we're not considering legislation issues here), it's not Level 4.

This is an important distinction. "Level 3 in most conditions" is good enough to pass as "full autonomy" for most people, but Level 4 is a requirement for not absolving the humans on board for any legal responsibility.