Am I the only one seeing the change between l4 and l5 abysmal? Also L5 description, at least in the Wikipedia article, is much more vague than the rest

In terms of convenience, it can be irrelevant.

However, in terms of technology, level 5 is a big step up.

Consider for instance that a level 4 system can assume that the ground is purely flat (an assumption that Tesla’s FSD vector space makes), while a level 5 system needs 3D information to navigate some vertically-diverse terrain.

There is a full document available on the SAE website with full descriptions and technical details. Its much better than the common summaries.

Still, Level 3 can be more "advanced" than Level 4.

How? Level 3 requires a driver who needs to take over when alerted. Level 4 is fully driverless.

No. It's fully driverless in geographically limited areas.

I'm aware. It's really between "maybe works, maybe doesn't everywhere (L3)" vs "works with no driver in a defined area (L4)". I consider the latter as more advanced as they are taking full responsibility for your safety.

It depends. If I could choose one car for person use, I'd take any modern adaptive cruise control + lane-keep assist system over an L4 that only worked in one city (even if it's a major city where I live). I'm not really sure how you determine which one is "more advanced," but I would consider the "level of automation" to be the portion of my normal driving habits that are able to be automated.

You're talking about personal driving, which is a different use case than robotaxis. For that, yes, you're better served with an ADAS system. It will take a while for L4 systems like Waymo to trickle down to passenger cars.

The point is that a "level 4 system" like Waymos is never going to trickle down to personally owned vehicles, because nobody wants to pay the hardware premium required for Waymo's autonomous system if it will be locked down to a specific city with specific routes.

Which is exactly GC's point – they don't want a car that can drive itself in Phoenix and SF and that's it. Most people either want a car that can make their life easier for a huge portion of driving but still require them to be in the driver's seat (e.g. Tesla's Autopilot) or a car that can actually drive itself basically anywhere (e.g. Tesla's FSD). Waymo's Level 4 just isn't that appealing for a personal vehicle.

This is why the SAE level system is poorly thought out and not particularly useful for anything except arguing over minutiae.

Any l4 autonomous system is capable of adaptive cruise control anywhere.