"adverse impact" is designed to sniff out hiring criteria that are a proxy for discrimination based off criteria such as race, gender, and age.

For instance, if you selected for college graduates for hiring firefighters, and that causes a lot of rejected applications among African American candidates, then it would be illegal based off of an adverse impact against a protected group.

As an aside, suing employers over requiring a college degree is a seriously under-tapped and under-estimating way to charitably improve the lives of Americans (college is a positional good because college matters when employers are looking at candidates - if employers can't look at whether or not you have a college degree unless it's a bona-fide qualification, then many more people don't have to spend several years and tens of thousands of dollars)

That's not nearly enough. The 4/5ths rule is designed to identify "structural discrimination", or cases where no discriminatory act can be identified, but the outcomes are nonetheless considered discriminatory. As long as a lawyer can show that some protected group is disadvantaged by the quiz, that's enough for it to violate the rule.