I am trying to understand a solution to a question: Let $X_1, X_2 ...$ be independent random variables with distribution $Normal(0, 4)$. Let $K$ be the smallest value of $k$ such that $X_k > 4$ (the index of the first $X_k$ that is greater than 4). Find $E(K)$ in terms of the standard Normal cumulative distribution function $\Phi$.
The solution reads that $K - 1 \sim Geometric(p)$, where $p = P(X_1 > 4) = P(X_1/2 > 2) = 1 - \Phi(2)$, which meant that $E(J) = \frac{1}{1 - \Phi(2)}$.
I understand why $K-1$ has a geometric distribution (We keep trying until one of the $X$ values $> 4$). I don't understand though why $p = P(X_1 > 4)$ (why 1 in particular) and why $P(X_1/2 > 2) = 1 - \Phi(2)$.