This is my first time breeding my yorkie, she is 3 years old, and 5 1/2 lbs. She should be 4 weeks pregnant, but I can't tell either way. I know all the regular signs to look for, tired, bigger nipples, vulva still a little swollen. But her tummy has not grown at all... is this normal. Should she not be getting bigger by now? Thanks.

You won't be able to tell at 4 weeks, with some dogs you can't tell at all until they go for their x-ray.

You can have an ultra-sound done now to confirm, or you can wait a few more weeks until the x-ray. X-rays can't be done until the later stages of the pregnancy when the pups bones start to calcify. Ultra sounds can be done earlier but are much less accurate as puppies can hide behind each other and you cannot get an accurate size of the litter.

Start mum on puppy food now just in case and start splitting her meals up into 2 or 3 smaller ones a day.
As the puppies grow, mum has less room for her stomach so large meals start to feel uncomfortable.

Of course, there is always a chance that the mating did not take, but it is better to risk fattening her up a bit for no reason that malnourishing the pups.

Because she is a yorkie, she might be pregnant with just two. Its not uncommon in small breeds and first litters. Both of those reasons will make it hard to see her round belly.

X-rays will be the only way to tell how many she has.