Mid-4s is where most people start feeling quakes, though the actual significance is pretty low. You'll tend to see/hear a lot of chatter online or in media.

At mag 5 there's localised damage, most characteristically of goods knocked off grocery store shelves, with glass-bottled liquids often producing a photogenic mess.

At mag 6, pre-code construction or at-risk areas (bay fill, river bottoms, sand) may see significant structural damage. The 2014 South Napa earthquake is the most recent of these, and downtown Napa was hit pretty hard, due to terrain (reclaimed river bottomlands, bay-fill, and some old masonry construction). See: <https://en.wikipedia.org/wiki/2014_South_Napa_earthquake>.

Mag 7 is the scale of some of the largest quakes recorded in California, including the 1906 San Francisco quake. This would create widespread damage within 100+ miles of the epicentre. Marc Reisner's A Dangerous Place (2003) includes a detailed description of impacts of a mag 7 quake along the Hayward fault, which would extend well beyond the immediate region into Southern California due to reliance on delta and Central Valley water projects.

<https://baynature.org/article/book-review-a-dangerous-place/>

Mag 8 is about the upper bound of expected seismic activity on the San Andreas and related fault systems.