I have not had my period for 3-4 weeks now, should i be concerned?

That depends on the: Lenght of your menstrual cycle? Or, are you saying that your period is 3 to 4 weeks late? Take a home pregnancy test if sexually active. Reasons for missed / late period include: an anovulatory cycle in which no egg is released, marked v or ^ in weight; eating disorder; over-exercising (endurance athletics), stress, travel, significant illnesses, oral contraceptives ; other meds, illegal drug use.