The magnitude of the source charge that generates an electric field of 1236 N/C at a distance of 4 m is 2.2 µC. This can be calculated using the formula for electric field, which is given by E = kq/r2, where E is the electric field, k is the Coulomb's constant, q is the charge of the point, and r is the distance from the point. Rearranging the formula to solve for q, we get q = Er2/k. Plugging in the values, we get q = (1236 N/C)(4 m)2/(8.99 × 109 Nm2/C2) = 2.2 × 10-6 C = 2.2 µC. Therefore, the magnitude of the source charge that generates an electric field of 1236 N/C at a distance of 4 m is 2.2 µC.