3 over 8 to the 4th power

(3/8)(3/8)(3/8)(3/8)=[3 x 3 x 3 x 3] / [8 x 8 x 8 x 8] =81 / 4,096=3/8^4=(0.375^4)

$(\dfrac{3}{8})^4=\dfrac{3^4}{8^4}=\dfrac{81}{4096}$

Same as Guest #1's answer.