Let x be the attendance in the fourth show to reach the average they want.
So to solve:
100 + 105 + 91 + x
----------------------- > 100
4
296 + x
--------------- > 100
4
Then, multiply both sides by 4 and the result is:
296 + x > 400
X > 400 – 296
X > 104 is the number to achieve the desired attendance average