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).
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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.
If a circle has a circumference of 43.98 the radius is: 7 (6.999).
If a circle has a circumference of 16 cm then the radius is about: 2.546 cm
The radius of a circle with a circumference of 30 inches is: 4.77 inches.
A circle with a radius of 9 inches has a circumference of 56.55 inches.