Why is my 4-month-old Shih Tzu's tail straight, not curved like others?

According to the American Kennel Club's standards, Shih Tzu's should have a tail that curves over the back. However, as long as your puppy is able to move her tail normally, i.e. Wag it, move it side to side, make it stand up, then there are likely no neurologic abnormalities associated with this issue. I think that at 4 months it's likely that her tail won't change much, but as long as you're not planning on making her a show dog, and as I said above assuming no neurologic abnormalities, she should make a great pet.

Talk to your vet on your next visit about your concerns. Especially if you're having trouble potty-training her, i.e. She's having trouble "holding it" for very long, there may be some concern for neurologic damage, since the nerves in that area perform multiple functions. Assuming she's progressing normally, I wouldn't worry about it too much, although you might touch base with the breeder, as this is likely something genetic. But lots of variations crop up during breeding - that doesn't mean they're "defects", just anomalies.

You might also look into contacting the Shih Tzu club in your area. The AKC's site (akc.org) can help you locate one.