Why do some doctors say that 4 ct n a year is not a lot of radiation but some say yes am i at high risk for a tumor no im 21 & all of the brain?

Dosage and age: While 4 ct's performed in a years time period is "not a lot" of radiation, it is always good to minimize exposure when possible. Radiation can damage dna, which sometimes can cause health problems. Another very important factor is age. I am much more concerned about small children's exposure than an adult because a child's cells are much more sensitive to radiation. It is always risk vs. Benefit.

Not high risk: Repeated ct scans do accumulate a moderate radiation dose to the area examined. There is some small increase in risk for radiation induced tumor. This risk is very small but should be a consideration when doing multiple repeat exams and must be weighed against the benefits. You are not at high risk for a brain tumor from those cts.