Senior year take Spanish 4 instead of AP Spanish, will hurt the admission of Harvard?
No; Harvard recommends 4 years of a single foreign language. They do not specify that is should be AP. Given that many schools, including elite private schools, no longer offer AP courses, it’s not surprising that applicants get admitted without any/many AP classes.
That said, there is much more to your application than just your schedule.
Thank you!
**TAKE AP SPANISH! ** If you do and score a 5 on the AP Spanish Exam, you will be exempt from Harvard’s Foreign Language graduation requirement. If you take Spanish 4, you’ll have to prove proficiency on one of Harvard’s placement exams or must take a year of Spanish if you are admitted. See: http://static.fas.harvard.edu/registrar/ugrad_handbook/current/chapter2/language_requirement.html
Virtually all US colleges give some kind of exemption or credit towards their college graduation requirements for a score of 4 or 5 on the AP Spanish Exam. So, if your high school gives you the option, take AP Spanish for that reason alone!
Or you can take the SAT subject test, which is a lot cheaper than the AP test 
Without looking at your total schedule, I could not best advise you which one to take. Obviously for Harvard, and schools of a similar caliber, you will want to ensure that you are taking the most rigorous courseload possible and are performing well in those classes.
Thank you both