Because she came from a petstore there is no telling where she came from or how big she will get. You could ask them but I doubt they know the size of her parents. Being that she is 4 pounds at 4 months, I would guess she would be at least 7 pounds. Of course, weight with yorkies is all relative, some have growth spurts in the beginning and then stop others just keep on growing and growing. Did they tell you whether or not she had any kind of health guarantee? That is something you'll need to think about, not saying al lpet shop pups are unhealthy but a lot of them are because they come from puppy mills. So, even if you get her for a good price now she could end up costing you a fortune in medical bills...or she could be perfectly fine, not knowing her genetic health in her family makes you have to take that risk.
It's really up to you, what kind of yorkie you are looking to get. for 300-500 dollars more you could get a champion sired yorkie within the standard meaning, ears erect, most likely guaranteed to be under 7 pounds, etc.
It's hard though, I've never been good at deciding things like that though, it's a lose, lose situation. I can't go into pet shopes because I want to buy all the poor dogs there and take them home!
I'll say one thing and then you'll see one that's the completel and total opposite of what you wanted and fall in love with it.
If I were you, I'd go see her if at all possible and let your heart decide on what you should do. If you really feel that she's the one then I am sure everything else will find a way of working itself out...
Sorry I couldn't be of more help...
Joy