Interview Question (#300!): Black and White

Sample Question #300 (probability brainteaser)
 
There are x white balls and y black balls in an opaque bag. (Obvisouly, you can’t peek into the bag.) There’re also a lot of white and black balls on the side. Each time you take a black ball out of the bag, you put a white ball (from the side) into the bag. If you take a white ball out of the bag, you do not replace the ball.
 
After you take out three balls, what’s the probability of getting a black ball on the 4th try?
 
(Of course, you may assume that both x and y are greater than 3.)
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