4. Wu Yu wrote three letters and three envelopes, and he needs to put each letter into the corresponding envelope, with only one letter per envelope. The number of all possible scenarios where at least one of the three letters is placed incorrectly is $\qquad$ kinds.
【Exam Point】Counting
【Difficulty】Standard

【Answer】 5
【Analysis】All possible ways of placing are $3 \times 2 \times 1=6$, with all placed correctly in 1 way, so the number of ways with at least one placed incorrectly is $6-1=5$.