L5 means the car can drive everywhere a human can. Waymo's refuse to drive outside of a constrained area, and occasionally stop to ask for assistance, so that makes them L4.

This whole autonomous driving levels kinda muddies the waters. Some would argue this isn't full L4 even. But it is a self driving car in the places it offers its services.

What would be the argument that Waymo is anything except L4?

No, L5 is a car that can drive itself anywhere in any conditions.

I think there's an implicit "where a decent human driver could drive safely" for L5, otherwise you get increasingly ridiculous scenarios like, "can Waymo drive safely in a whiteout blizzard?" or "can Waymo safely escape an erupting volcano??"