【Analysis】As shown in the figure, 1 person can exclude up to 3 seats $20 \div 3=6 \cdots 2$, therefore, at least $6+1=7$ people are needed.
4、A row of 20 seats, some of which are already occupied. If one more person sits down, there will be two adjacent seats occupied. The minimum number of people already seated is
$\qquad$ people