There are so many possible reasons. When I'm messaging with someone I have met on a dating site, there are all kinds of things I look for in those first few messages-- correct grammar, a sense of humor, shared interests/hobbies, education level, solid job, not religious, not a "partyer," likes and wants children, a sense of honesty and good values-- and so much more. I'm also seeing if someone can hold my interest by coming up with witty or sarcastic things to say, or funny stories to tell. There are also things that might instantly turn me off that I couldn't anticipate (i.e. doesn't like to read, thinks politics is "stupid," writes "lol" too much, badmounths their ex or other people in general, etc). Everyone has their own pet peeves, deal breakers, likes and dislikes.

It also could be that the girl was talking to four different guys about hanging out, she went a date with one of those guys, and really liked him-- therefore, she no longer is interested in meeting you (or the other guys she was talking to). Or, alternatively, she could have gone on 4 bad dates with 4 different guys and just needs a break from dating, period, right now.

Who knows? It happens ALL the time! People on online dating sites are notoriously flakey-- you can have a date at 7pm with the person and, at 6pm, they might change their mind. Until you have that first date-- and it goes well for BOTH of you-- you just can't put too much into the other person. People's commitment level in their RL just doesn't come through via online dating-- she may be totally reliable at work and with her friends, but on okcupid? Just don't expect much until you've met in person at least a couple of times and you know you're on the same page.