Catching a traffic light change in four seconds

A traffic light cycles green, then yellow, then red, then back to green. It is green for $40$ seconds, yellow for $4$ seconds and red for $40$ seconds. You look at it during a $4$-second window starting at a uniformly random moment. What is the probability that the colour changes during your window?

$\tfrac17$. A change is seen exactly when the window starts within the last 4 seconds of whichever colour is showing. That is $4$ seconds per phase, three phases, out of an $84$-second cycle: $\tfrac{12}{84}=\tfrac17$.