Here were see that the coefficient of x is -3. In order to find a line that is perpendicular to another line, we need to put the opposite sign and make the coefficient the reciprical.
So now we know that y=1/3x will be perpendicular to your line. If the point goes through (0,4) that means when x is zero, y is 4. If we plug that into our expression 4=1/3*0, we see that we need to add 4 to make this true.
