My new yorkie is at 5.5 lbs and now turning 6 months (on the 15th) he hasn't gained weight since I got him a month ago. He was produced using 2 different pure lines (CamparisxJoymason) maybe that's why he's huge.
<--here's his photo
My first yorkie, reeses, was 4 lbs at 4 months, now he's 1 year and 5 months at 5.5 lbs.
My girl was just 800grams at 4 months, she's now 1 year and 2 months at 4lbs.
The weight and growth chart is a nice guide for guesstimates... but just as others say, it really depends on the genes and health (some illness stunts).