4 month old has been fighting an on/off fever for 2 weeks with no other symptoms I can see. At what point do we call the dr?

During office hours. Fever, usually defined as a temperature above 100.4 f or 38 c, is usually the body's response to an infection. A 4 month old can be sick but have few symptoms. If your baby has been having a fever for 2 weeks, he should be seen. Call the doctor to make an appointment.
Now. About 8-10 days ago. An elevated temperature longer than 3-4 days should be evaluated for cause. Time to go.