Growth Varies by Puppy
A four-month-old Border Collie being smaller than others does not necessarily mean she won't reach normal adult size. Growth rates vary widely.
Factors to Consider
Breed standards, genetics, nutrition, and health all influence size. If you are concerned, consult your veterinarian to check her growth curve and rule out medical issues.
What to Do
Monitor her weight, provide high-quality puppy food, and stay patient. She may still have a growth spurt later.