2 I TeXed the mathematics.

From the Markov property of the random walk (X_n) $(X_n)$ we have

P(X_4>0

$$P(X_4>0 \ |\ X_3>0, X_2>0) = P(X_4>0\ |X_3>0).\ X_3>0).$$

To paraphrase Kai Lai Chung in his book "Green, Brown, and Probability",

"The Markov property means that the past has no after-effect on the future when the present is known; but beware, big mistakes have been made through misunderstanding the exact meaning of the words "when the present is known"."