A 4lb dog is still tiny. I understand that it is upsetting that the puppy was not correctly represented, but it is yours, and we all live and learn from our expierences.
I also wanted to add that a good breeder shouldn't necessarily charge alot more money for a tiny yorkie. Maddie, my little girl is 2 1/2 lbs and alot more work than any dog I have ever had. The breeder I got her from knew that the small ones take extra care and sold her to me for about 600 dollars less than what she sold Maddie's bigger brothers. Mad's sister is slightly smaller than her and sold for 300 less than Maddie.
I would just enjoy my new puppy if I were you, I am sure that she is just precious. We love puppy pictures here on YT!
Oh and Welcome.