An arc is a portion of the circumference which is 2pr. And you want the difference between the large and small arcs..
a=2pr(120/360) (we are multiplying the circumference by a fraction of the angle vs a complete circle...)
And the difference we seek is between the large and small arcs so:
d=(2p/3)(R-r)
d=(2p/3)(5-4)
d=2p/3 cm
d≈2.1 cm (to nearest tenth of a centimeter)
