4 is the circle's diameter.
The radius of a circle is half of the half of a circle. So, imagine having a line cut through a circle (the diameter). Half of that line is called the radius. In mathematical format, the diameter over two, or divided by two, is the radius.
Diameter is the radius times 2, you can visualize it as a straight line cutting a circle in half.
Now, knowing this information, if the radius is two inches, all you have to do is 2*2, which is 4.
In conclusion, the answer would be 4 inches, or 48 centimeters if you want to get specific.