It's actually debatable that you ever need level 5, because once you reach level 4 it's probably only a matter of time until most road infrastructure is converted for L4 self-driving vehicules (once you can statistically prove a self-driving road kills 1000x less than one with human drivers).

Like we never wondered about self-driving trains, essentially. It just became a fact once we could do remotely supervized L4, and that novelty was short-lived too.

taxis in a big city isnt neccessarily level 4. True, theres no person in the drivers seat, but there are still things that restrict the operation of a Level 4 system, while 5 is supposed to go anywhere, anytime. Snow, Rain, Fog, even night could all impede Level 4 systems depending on the implementation's restrictions.