Would Harvard understand that I got 4 B’s because I transferred to a new school in the middle of the year but end high school with a 4.35 GPA?
What do you mean “understand”? Yes, I think they know what a B is, and would be aware of your circumstances, assuming you provide transcripts from two different schools.
I wouldn’t suggest trying to “explain” it with an “excuse”.
Yes, people do get accepted into Harvard with B’s. They will take them in the context of your class rank (if your school shares yours) and the difficulty of the classes you took. They will likely recalculate your unweighted GPA using their own method, so as to compare apples-to-apples across most schools. Do keep in mind, though, that competition is fierce, and many applicants will have perfect transcripts. Take your shot, but be sure to apply to a few schools where your chances are multiples higher than 5% or so.