Please don't take it as anyone scolding you. It's just that you may have walked into a possibly life threatening situation for your new girl. If she is only 4 pounds and nearly ready to whelp, she may very well be too small to safely deliver & require a ceserean. An X-ray will help determine if there are pups, how many and whether her pelvis is wide enough to deliver.
Also, a breeder who is so irresponsible as to sell you a possibly whelping female is also most likely to have not done any of the necessary pre-breeding health tests on either the male or female
There is no need to go to an emergency vet tomorrow (unless there IS an emergency) but on Monday morning you do need to call and explain the whole situation to the vets office. If they still can't get you in, they aren't the kind of people you would want to have taking care of your baby on a regular basis anyway. There are lots of vets out there, just call another office.
There are a lot of supplies you are going to need to start collecting in a hurry if she is whelping. The most important thing to find is an experienced mentor. Try calling any rescues in your area and see if they can put you in touch with someone who would be willing to help.
For your sake, I do hope this is a false alarm and you can just enjoy being a new yorkie mom. But if it's not, you'll get lots of good advice here. It won't necessarily be sugar coated, but it will ALWAYS be with the best interest of your dog in mind.