By Newton's second law of motion, we know that the resultant force acting on a body is directly proportional to the mass of the body and directly proportional to its acceleration. In system international (SI) units, the value of the constant of proportionality constant is 1. Therefore, the equation for Newton's second law of motion becomes: F = ma, where F is the resultant force, m is the mass and a is the acceleration of the object. Substituting the values of m and a into this formula, we get the result: F = 12 x 4 = 48. The SI unit for force is the Newton; therefore, the answer is 48 Newtons.