Interestingly, often, though they can seem more expensive, private colleges tend to have higher four-year graduation percentages:
"Private schools do a better job, in general, with graduating students. The average 4-year graduation rate of our list of private schools was 59% while at publics it was 32%. Beyond that, the vast majority of those that do graduate from private schools do so in four years (over 80%), while almost half of graduating students at public schools take five or more. Arizona State University, the largest school in our list, graduates 57% of its students in 6 years according to 2011 U.S. Department of Education statistics. Of those that graduate, roughly 44% take longer than four years."