I'm planning on taking some courses this summer, but based on what I've seen, 4 would be difficult. Due to the way scheduling for summer courses is set up, it's not really feasible to take more than 3, and even then it would be a tricky course load, and your options would be very limited. From what I understand, it's common to take some of the more general courses before starting the program. For example, taking the calculus sequence at a community college in order to reduce the number of courses you need once you get to BU.