The first seat can be any one of the 4 people.
For each of those . . .
The second seat can be any one of the remaining 3 people.
For each of those . . .
The third seat can be either one of the remaining 2 people.
For each of those . . .
The fourth seat is the last remaining person.
Total number of possible ways to line them up = (4 x 3 x 2 x 1) = 24 ways.
