Probability that a ball drawn after a random transfer is white

There are two urns $U_1$ and $U_2$. $U_1$ contains four white and four black balls, and $U_2$ is empty. Four balls are drawn at random from $U_1$ and transferred to $U_2$. Then a ball is drawn at random from $U_2$. The probability that the ball drawn from $U_2$ is white is