Answer
$|z|=4$
see graph below
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Work Step by Step
See p. 602, Graphing Complex Numbers
The complex plane is determined by the real axis and the imaginary axis.
To graph the complex number a + bi, we plot the ordered pair of numbers (a,b) in this plane
See p. 603,
The modulus (or absolute value) of the complex number $z=a+bi$ is
$|z|=\sqrt{a^{2}+b^{2}}$
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$z=4i=0+4i$
$(a,b)=(0,4)$
Plot (0,4) in the complex plane (see image).
$|z|=\sqrt{0^{2}+4^{2}}=4$