This another special case much like real numbers. If we were to use (4) to find the argument we would run into problems since the real part is zero and this would give division by zero. However, all we need to do to get the argument is think about where this complex number is in the complex plane. In the complex plane purely imaginary numbers are either on the positive y-axis or the negative y-axis depending on the sign of the imaginary part.
For our case the imaginary part is positive and so this complex number will be on the positive y-axis. Therefore, the principal value and the general argument for this complex number is,