Lots of nice day hikes in the Phoenix area, but really…Phoenix is a driving town. And it’s not CA…
Agree with @Marilyn about San Diego, but really, they will likely spend as much on Uber and Lyft as they would renting a car (I made that mistake in Austin…really renting a car would have been cheaper).
Is there any reason why they can’t rent a car? It would open up so many more options.
If they are traveling from the east coast…in my opinion, four days isn’t enough time. They will be spending two of those days traveling. That would only leave two days in CA.