I was actually researching this myself over the last week or two for my daughter. I told her she should probably seriously think about continuing through a 4th year, based on some of these from admissions officers:
“[. . .] the student studying for four years has a genuine interest in knowledge and education, not just in fulfilling minimum foreign language requirements.” – Matthew Potts, Admissions Counselor, University of Notre Dame
“We give the most consideration to students who have taken the highest level language available at their school.” – Robert Killion, Office of Admissions, Haverford College
“Our recommendation is that, in terms of high school preparation, students take 4 years of a single language” – Steve LeMenager, Director of Admissions, Princeton University
“[. . .] the more years of a language, the better—it shows that the student has gone beyond the minimum requirement.” – Lia Brassord, Assistant Director of Admissions, Smith College
Admission Officers are making distinctions between thousands of overly qualified applicants. In speaking with students about senior year course selection, we encourage students to think carefully before dropping a language.” – Andrea Thomas, Assistant Dean of Admission, Hamilton College
[. . .] depth and mastery are important in the serious study of any discipline. The student who is willing to do more than the minimum is always more appealing.” – Ray Brown, Dean of Admissions, Texas Christian University
“Ideally a student will present at least 4 years of the same language (classical or modern) if the curriculum allows.” – Terry Cowdrey, Dean of Admissions and Financial Aid, St. Lawrence University
“The more years in one language the better it shows commitment and desire for proficiency.” – Dennis O’Driscoll, Director of Admissions, Creighton University
“Three to four years of a language shows follow-through and a deeper level of interest.” – Michael C. Behnke, Vice President for Enrollment, University of Chicago