I have a sore throat for 4 weeks now and they said it was Strep type B but now it's negative. And I still have a sore throat. Could it be mono?

Yes, possibly. Group B strep rarely causes sore throat. GBS often resides normally in the throat so positive test didn't mean it was causing your symptoms, which probably were due to a virus, not GBS. Mononucleosis is one possible cause of prolonged sore throat, but usually there are other symptoms too, like fever, lymph node enlargement, and fatigue. Discuss with your doctor, maybe have a test for mono.