I live in NJ and am familiar with all these schools. One differentiator is that Delbarton has a religious affiliation and is all boys - the others are co-ed and secular. If all boys and or catholic are important then your decision is made - if you don’t want that then you can narrow it down to the other three.
What sports is your son interested in - although all four have what I would call strong sports programs - they definitely have differences in which sports they excel in. So that would be the next criteria - for example - Delbarton is strong in LAX, wrestling, Pingry soccer and LAX, not sure about the other two, but it can be easily researched (just check their season records on NJ.COM).
Anecdotally, I know a strong Delbarton family (Dad and two sons all went). Both boys played sports (assume the father did also) and attended top colleges. Know a few kids who went to Pingry. They participated in the theater program (I saw a couple of the productions - very good) - again attended top colleges.