The circumference of the circle is given by pi diameter so we have pi*2 for 1 circumference . the portion of 2*pi circumference for the whole circle for this arc is (4/2*pi).
The total angular measure of the circle is also 2*pi(or 360 degree)
So, we the measure radian of the angle is ((4/(2*pi))*(2*pi)
= 4 radians