Question on confidence intervals for uniform distribution

I have a random variable $X$ which is distributed uniformly over $[0,b]$. I want to know based on only one observation what is the confidence level of the interval $[X, 4X]$? I know this entails finding $P(X \leq b \leq 4X)$ but am entirely unsure how to approach the question. I have only had experience constructing confidence intervals for normally distributed variable and am very thrown off by the fact that $X$ is distributed uniformly.

It's always the case that $X \leq b$, so you only need to find
since $X / b \sim$ uniform$[0, 1]$.