I found this, so perhaps it is just a reaction to the vaccine and everything will be fine. At least, I hope so. I know you said it was four days ago, but some of these babies are so small that time may not mean that much.
What about vaccine reactions?
It is true that some animals have a systemic reaction,
including a low-grade fever or muscle aches and pain. This reaction is more common in young and toy breed dogs and causes them to eat less and sleep more for 24-48 hours. Rarely, dogs will have a more severe reaction, characterized by hives, swelling of the face, or even vomiting. This reaction is easily prevented by giving antihistamine at the time of subsequent vaccinations. Leptospirosis, the component most likely to produce such strong reactions, can be left out of some vaccines. If your dog has had a vaccine reaction in the past, don't skip future vaccinations but do warn the veterinarian so he can take steps to prevent a recurrence. http://www.canismajor.com/dog/vaccine.html