He looks all Yorkie to me, and I was thinking the same thing about your avatar. Usually Yorkie mixes are bigger than 4lbs. What the heck did they breed him with if he's a mix? Isn't a Chi the only thing that could be smaller than a Yorkie? And he doesn't look like he has Chi in him.
Maybe one parent was 1/2 Yorkie, so he's really 3/4 Yorkie.
Who told you he was mixed?