I took 8 AP classes between sophomore and junior year. I got a few 5s but mostly 4s. However, I got one 2 on a test. If I am applying to highly selective schools (Harvard, Stanford, MIT), would it be wise to self-report the 2 on common app or will colleges be suspicious that I took the class and didn’t submit a score? Also, are 4s high enough for these types of colleges?
Do NOT report the 2, as it is not a passing grade and will likely not give you credit or placement at most schools. Colleges will not question one not reported score out of the other tests you have taken (it’s especially common for students planning to retake courses at the college level or in classes not related to their intended major to not report the score.) It’s better that the college (hopefully) believes you didn’t take the test vs. giving them irrefutable evidence that you took the exam and didn’t pass.
The 4s question is harder to answer. Is it possible to get into T20s (I didn’t apply to HYPSM, so can’t speak to their higher admissions standards) without 5s, even in subjects related to your intended major/career? Yes. Do 5s look better? Also yes. To what extent? No one but the Admissions office knows for certain, and holistic admissions doesn’t give any particular weight to one type of test score vs. another.
From your post, I’m assuming you’re a Junior/Senior rn, and short of retaking the tests, there’s not much you can do to change your AP scores. Selective admissions are far from a guarantee for anyone and are dependent on a variety of factors, many beyond your control, so apply broadly and hope for the best. 
Hope that helps! Good luck with admissions!
AP scores have minimal impact on admissions - the grade in the course is much more important. AP exams/scores are primarily for placement (hence the name) after being admitted. An AO of a T20 told a parents group at our school that he doesn’t even get AP scores in the info packet he uses for admissions review/selection.
Don’t send the 2, send 4s and 5s - admissions staff isn’t going to do detective work matching up courses and tests.
@PikachuRocks15 Thank you for informative reply! I feel bad that you wrote so much for a random like me. Nevertheless, it is greatly appreciated.
@RichInPitt Thank you for the response. I feel much more relieved!
No problem! The information on CC was extremely helpful to me while applying this past cycle, so I’m happy to help!