I do understand what mutually exclusive events mean (i.e. $ A \cap B = \emptyset$). But I came across this example in this book
The following table presents probabilities for the number of times that a certain computer system will crash in the course of a week. Let A be the event that there are more than two crashes during the week, and let B be the event that the system crashes at least once. Find a sample space. Then find the subsets of the sample space that correspond to the events A and B. Then find P(A) and P(B).
and the author states that
The events “3 crashes happen” and “4 crashes happen” are mutually exclusive.
Why these events are mutually exclusive? If a "4 crashes happen" event happens means (to me) the "3 crashes happen" event happens for sure. Am I right?