We know that the relationship between Electric Field , Charge and Distance is given by :

Where, k
Columb's Constant

Q= Source charge that generates the electric field
R = Distance from the charge =4m
E=Electric field = 1236N/C
As, we have to find the charge therefore rearranging the formula we get :
Putting the values we get

On solving
µC
Therefore the magnitude of source charge is 2.2 µC