Form the pictures I googled, it seems that the trams at DFW are rail-guided, although they ride on rubber wheels. But I keep reading that this place or that is using true autonomous vehicles for local shuttles.

The boundary between level 3 and level 4 is quite clear, as I understand it. In level 3, there must be a driver behind the wheel. Waymo and others hit level 4 when they took out the safety drivers. The boundary between level 4 and level 5 is that level 4 has geofencing, weather, and possibly other restrictions. Level 5 is equivalent to a human driver. But I think the guys like Waymo who are doing level 4 on city streets today all have a central monitoring station where operators keep an eye on all cars all the time, and can intervene when necessary. I think that makes the boundary between 3 and 4 a bit squishy. I think the monitoring intervention is more for if a vehicle gets stuck in some situation, as they seem to from time to time, rather than real-time collision avoidance.