Been smoking for a year plus and quit 4 days ago. been having sore throat and fever since. is it nicotine withdrawal symptoms or bacteria infections?

Not withdrawal: Nicotine withdrawal has been known to cause quite a few problems but not sore throat and fever. This is likely to be an infection, but whether it is a bacterial or viral infection, only your doctor can tell you after taking your history and examining you, and possibly doing some tests.

Not nicotine....: A sore throat and fever is likely due to either a viral or bacterial infection. A viral infection typically gives a low-grade fever whereas a bacterial infection will usually give a higher fever but there is considerable overlap. See your doctor if your symptoms are worsening over the next few days. Congrats on quitting smoking, one of the best things to do for your health!