Explanation:
you do understand that the diameter of a circle is simply 2 times the radius, yes ?
imagine a circle and you draw a radius line (from the center of the circle to the circle arc).
and now imagine to draw another radius line, just in the opposite direction.
when you look at the result, you have 1 straight line going from the circle arc to the circle arc and passing through the center of the circle. that is what a diameter is.
how long is it ? well, we drew 2 radius lines to create this line. so, it is 2×radius long.
so, the diameter of the seal is
2×2 = 4 yards