[Faboosh]

The rule I usually see is whatever weight they are at 12 weeks, double it & that's usually their adult size- give or take a lil bit. If you google yorkie growth chart (isn't there one on YT??), they break it down by the week & ounce.

So it sounds like your girl is "charting" to be around 4 lbs... tho that's just her weight, not her size, like physical structure. Post pix of the cutie!