I also am worried. Four-month-old puppies and kittens should grow and gain weight every day. When they fail to do this, and especially if they lose weight, something generally is wrong.
If your puppy is eating well and still losing weight she may be suffering from a heavy parasite infestation. Roundworms are endemic in puppies and kittens. Severe infestations can cause weight loss or failure to thrive. Infestation with microscopic parasites called coccidia also may compromise growth.
A number of other issues can cause poor growth or weight loss in puppies or kittens. These include developmental or hereditary anomalies, bacterial or viral infections, malnutrition, foreign objects lodged in the intestinal tract, and an intestinal anomaly called intussusception.
Your puppy needs to see a vet to identify the cause of the problem. Hopefully a deworming will solve the issue.