Visa requirement for Summer Term

I’m interested in taking the Computer Networks Course which is worth 4 credits at Boston University. However, with only 4 credits - I’m eligible as a Part-time student at the university. And I won’t be issued an I-20 form for getting an F1 student visa.
I cannot enrol for 8 credits as no other course really fits my interest as of now. Therefore, I wanted to know what are the risks of travelling on a B1/B2 visitor visa, staying on campus and taking a summer term course. Also, what’s the difference between a Part-time student and a full-time student.

https://www.bu.edu/summer/international-students/visa-information.shtml