[jean_tta]

More broadly most applicants apply to several schools. If there are as many spots as applicants and each applicants apply to k schools, you could go as low as 1/k average admittance percentage (rates will be higher when colleges accept applicants that do not come: you could have 100% rate after all). With k = 12, you're at about a 8% lower bound for the average college.

For elite colleges that will attract more applications and will have fewer admitted-but-does-not-come applicants, 4% is not that low.

If the marginal cost of an extra application is lowering (say, because we went from typewriting & mailing everything to copy-pasting & online applications), the number of applications per applicant will go up and thus the admission rate will go down.