As other posters have said, one can get into an excellent graduate program anywhere in the US from these four schools. That wouldn’t be one of my deciding factors when choosing between them.
One has to be very careful with graduate school placement data, especially PhD production. The success rates of applicants matter much more than the overall numbers – Reed may produce more PhDs per capita than Penn, for example, but that does not mean that any given student from Reed has a better shot at (or is better prepared for) a PhD program than a student from Penn. It simply means that more students at Reed are interested in a career in academia.
As for Duke specifically, students tend to be much more interested in professional degrees (medicine, law, business) than PhD programs, but the students who do apply to PhD programs typically fare quite well.
I don’t know how long ago that was, but that was not at all my experience. It’s not that unusual for Duke students to hit up Franklin Street or a concert at Cat’s Cradle in Carrboro. You also had Carolina students coming over to Duke/Durham for shows at DPAC or parties/concerts on campus like LDOC. There’s a fair amount of social intermixing between the two universities, and the Robertson bus makes it really easy to travel between them.
I took several classes at Carolina and sometimes wore a Duke shirt. The only comment I ever got was someone asking me if I’d lost a bet.
To judge from past posts, OP was (is?) interested in majoring in biology. If that’s the case, it may be useful to compare course offerings at each school.
JHU’s biology department, for example, has a relatively narrow focus on biochemistry and molecular/cellular biology. Duke’s biology department embraces many other subfields in addition to biochemistry and molecular bio – marine biology (with a marine lab on the coast), ecology and environmental biology (with an 8000 acre research forest), primatology/biological anthropology (with a primate center), botany and plant systematics, and so on.
https://biology.duke.edu/undergraduate/major/concentrations
I am less familiar with the biology programs at Williams and Pomona, but it’s my impression that they have relatively well-rounded biology departments.