Those who do AP courses also do the four years, SeaToSki. A lot of universities will allow a student to do 150 or 200 level courses in the AP subjects if they have attained a 4 or 5 in AP exams, but some disregard APs.
Most highly selective universities have a Core curriculum which all students admitted must follow for at least part of the first two years, often alongside courses that are part of their major.
The four year university system is not just to allow weaker students to catch up. The philosophy of breadth, of the well-educated citizen, and the importance of a common vocabulary is expressed in the American education concept - hence the broad curriculum in high school and also the four years in university where a broad curriculum is the aim too - even universities that are not Ivy Leagues can require that all students pass a certain level of English, math, science, mfl and humanities courses before they can graduate.
Here is a succinct idea of what core coursework aims at, in this case from the University of Chicago's website.
collegeadmissions.uchicago.edu/academics/core
A University of Chicago education is more than a set of skills, a rite of passage, or even the ability to think critically. It is an experience—part of a lifelong dialogue that encourages students to converse across cultures and disciplines—and is guided by 75 years of experience with a Core tradition.
This famed Core curriculum, a model for American general education, is the University of Chicago student’s introduction to the tools of inquiry used in every discipline—science, mathematics, humanities, and social sciences. The goal is not just to transfer knowledge, but to raise fundamental questions and become familiar with the powerful ideas that shape our society.
Not only does the curriculum provide the background for any major and for continuing study after graduation, it also provides a common experience for all students in the College. All students have taken the same sorts of classes and read the same kinds of texts, struggling and triumphing over the same sorts of ideas. This gives every student a common vocabulary of ideas and skills, no matter his or her background before coming to the College.
collegeadmissions.uchicago.edu/sites/default/files/uploads/pdfs/brochure_core.pdf
A vital tenet of the Core is that interdisciplinary collaboration can illuminate complex problems—and that this exploration is best achieved when students are well-versed in multiple disciplines.
The Core offers broad exposure to the arts and sciences so that all students gain a multitude of perspectives regardless of their major.
Cross fertilising of ideas is another important ideal behind the American system. (This is an ideal not shared by the UK education system.)