First, I'd like to say that this question is flawed because the diameter of the spool changes as you pull the line out. Some would argue it's negligible I suppose.
At any rate, assuming there's a magic spool where the diameter doesn't change, let's find the cicumference so we can find the length of one wrap around the spool.
circumference = 2*pi*r = 2 * pi * 4cm = about 25.133 cm
Now if it turns 16 times we'll have 16 times the circumference.
16 * (25.133 cm)
= 402.128 cm