If you're working with complex numbers, then I'm sure you're comfortable with plotting them on a complex-plane ... real part of the number along the x-axis, and imaginary part of the number along the y-axis.
When you look at it that way, your two points are simply two points on the x-y plane:
4 - i ===> (4, -1)
-2 + 3i ===> (-2, 3) .
The distance between them is
D = √ (difference in 'x')² + (difference in 'y')²
= √ (6)² + (4)²
= √ (36 + 16)
= √ (52)
= 7.211 (rounded)