Answer: 840
Explanation:
Given : The total number of empty seats in the theater = 7
The number of customers need to find places to sit = 4
Since here order of their sitting matters , then we use permutation to find the number of ways of sitting.
The number of permutations of n things taking r at a time is given by :-

Then , the number of permutations of 7 things taking at a time is given by :-

Hence, the number of different ways can these four seat themselves = 840