My Yorkie did in fact have a "line" down his back when he started changing colors, and yep...it was right around 4 months of age!
We called him our little skunk lol
The line going down his back wasn't white though, it was silver as this is the color that he turned (although really it is more of a dark gray, right now away). Is there any way that you could go back to the breeder and take some pictures? Just tell her that you want to show (someone) the little pup's pictures and then maybe you can come back and post them here for us to see so that we can give you our opinions? I'm sure the pup is a purebred Yorkie, but it would be nice to see some pics just to be sure
Good luck in whatever you choose to do!