Quitting an unpaid internship?

[concerned]

So currently I am a junior in college and accepted an unpaid internship for this semester. Well, I have gotten through 4 days of it and want to leave already (its that bad). And I have stayed with jobs I HATED for years before. Right now I am taking 5 classes, and this UNPAID internship is asking 20 hours a week (on top of my paid job). What really gets me is the boss though. As far as I can tell the company has only 2 paid full time employees and relies completely on unpaid interns otherwise (which to me seems like a scam, there are 4 of us). This is a local-based PR/social media managing firm.
The boss is possibly the most stuck up, nasty, unpleasant woman I have ever come across. She sneers at everyone, curses all day stomping around the office, and tries to make you feel like an idiot constantly.
On top of all this, I am feeling over committed and stressed from my heavy work load.
everything I have read on the internet says quitting an internship is the worst possible thing you can do, but I cannot see myself sticking this out and think its better to call it quits sooner rather than later.
HELP!!!!
also: i am not receiving college credits for this either!!! this internship was described as "for experience only" so it will not hurt my academic criteria at all. basically i am worried about future job prospects, but I figure what are the odds that anyone in 2 years will remember an intern who worked there for 4 days. Also obviously would not include on my resume.