I would guess that the difficulty level of Korean on duolingo is greater than four semesters at a undergraduate college level at an American University

It is really really hard

Duolingo does make some mistakes but you can save a lot of money completing a class on duolingo before you hire tutors or take a college class because it goes up to college level difficulty

Eventually if you want to know the language well you will need a source other than duolingo but it provides a very advanced start