Hmn...I do not agree with anyone here. The past, though not always, can be good indication of the future. History does tend to repeat itself. 4 people just in college is not a small number for a girl - one dude per year. I say for a girl, because girls are naturally supposed to be less promiscuous than guys, if anything, because the outcome of getting pregnant for a girl caries a heavier toll.
I don't know much about your relationship, but you have a good reason to question this. A guy's worst nightmare is ending up with a "slut". Guys will have sex with a girl like that, they will respond to her, but no guy has respect for a girl like that.
I'm not saying she is like this, but if she is, the ramifications of her being like that at the least spell the end of your relationship. You have to really think about this.