It is extremely unlikely that your dog has been exposed to a strain of the parvo virus to which she has no immunity, so I would not be concerned about that. I would be concerned if she is throwing up so much that she is becoming dehydrated, she is vomiting and also has behavioral changes (like depression), or if she has diarrhea (you already mention gassy).
It sounds like garbage gut gastritis, but there is no way to diagnose this without an exam and sometimes blood work and xrays. A four month old puppy is old enough to hold off of food for a day. If the vomiting continues even when she is not eating, or if she starts vomiting again as soon as you put her back on the food, she will need to see your regular vet.