You neglected to say whether the 4mm is the radius of the circle, or the diameter,
or the circumference. Each possibility leads to a different area.
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Pie (3.14159...) times (*) 2 times (*) radius === or This equation may be for the area but hopefully it's for circumference. Anyway, the answer to your question is 25.13274123... (Pie times 2 times 4mm).
151200
Area of a circle = Pi * radius2
the area of the circle is increased by 400%
Area of a circle = pi*radius2