The term PREFIX WITH 'BALLISTICS' OR 'MAGNETIC' can be solved by the word AERO, which is a four-letter prefix. AERO is derived from the Greek word aer, meaning air or atmosphere. It is commonly used in scientific terms to refer to air or atmospheric related phenomena, such as aerodynamics, aeronautics, and aerobatics. When combined with the words 'ballistics' or 'MAGNETIC', it can refer to the study of the motion of projectiles through the air or the study of MAGNETIC fields in the atmosphere.