This is retarded. Your chick is a lying, cheating wh0re and you wanna "sit her down?" In FOUR DAYS?! Brother... call her right NOW (or if the line is busy, text her): "i'm dumping you like yesterday's garbage. You're now somebody else's problem. Stay the fvck out of my life.
Exactly, get rid of her, now.
It's one thing if she said 'hey, i want to hang out with some friends, my x will probably be there, are you o.k. with this?', but what she is doing is SHADY bro, dump her and haul a$$.
I like Krassus's approach, call her and be a total d*ck to her, but in a calm and cool way, you dig?
Good luck...