English Toy Spaniel comes to mind too....another small dog. They also have a "breed" of Shih Tzu called a "Chinese Imperial" or "Imperial Shih Tzu" that are supposed to be between 4 and 9 lbs. (I've only seen these on the web, so I may be putting my foot in my mouth in the same way people do when they say "teacup" Yorkie, but I don't know.....I only know what I've read about them.) Shih Tzu's are small in size, but they weigh quite a bit more than standard Yorkies. I thought they were terribly pretty dogs, which is why I looked at them and then more closely at the Chinese Imperials. BUT, I fallen in love with the breed (Yorkies) and will be getting another one, myself, when the time is right!
Good luck on your search!
(There is a website that has pretty good breed descriptions including size/weight, and it's actually where I found Bella....I won't do that again now that I know what I know, but it's still a good resource to search breeds and see pictures. It's
Puppies for Sale, Dogs for Sale and Dog Breeders if you're interested. On the left side of the page, select "Small" from the drop down and "Search" [nothing comes up when you search "X-Small"]. That will get you a pretty big list of small dogs and descriptions of the breeds.)