It can work, but just not like that.
For example, if you were with a girl that you were already having sex with and she was being stingy with the vag because she was mad at you over something minor, you could pull the "well if we aren't going to do anything I'm going home card." and that can work.
The reason it can work is because you aren't really giving her a threat that she feels like/feels comfortable making a HUGE deal of. If you said some **** like "give it up, or we are over", chances are she will call you an ******* and probably break up with you. At the same time, the idea of you being angry with her is generally enough to convince her that the situation isn't worth throwing a fit over.
HOWEVER
It will not work with a girl who hasn't given it to you yet because she knows that she is still holding all of the cards. No matter how she plays herself off to you, she knows that you want the vag. In fact, in your case she knows that you want the vag bad enough that you were willing to go through four months without it.
That is, unless she is still a virgin. Thats a whole new can of worms. With that, pressure will get you nowhere but nexted.
In either case, if you haven't had it yet, you need to romance the ***** a little bit. Set the mood, and just let the time feel "right" like she wanted. Think of it as like convincing somebody to tell you a big secret. She has to feel comfortable.
That means taking her to the backseat of your car and putting some slow jamz on after everybody has passed out at your buddy's kegger will not work.
Be inventive.
Good luck