The purpose of a 4 year degree and a PhD is very different. Roughly speaking, coming out of a 4 year degree doesn't signal expertise in anything. It means you

The purpose of a 4 year degree and a PhD is very different. Roughly speaking, coming out of a 4 year degree doesn't signal expertise in anything. It means you have learned to "think like an X" to some degree, and have a working knowledge of some of the foundational stuff in an an area. Coming out of a PhD signals you have achieved expertise in a pretty narrow area of a subfield (and explored the bleeding edge a bit), but also that you have learned how to gain that sort of mastery again, alone, in at least related areas.