We can use the following formula to solve this problem:
men × hours × length of wall = constant
We know that six men can build a 150 m wall in ten hours. Therefore, we can write:
6 × 10 × 150 = constant
Simplifying this equation, we get:
constant = 9000
Now, we can use this constant to find the number of men needed to build a 200 m wall in four hours. We can write:
men × 4 × 200 = 9000
Simplifying this equation, we get:
men = 9000 / (4 × 200) = 11.25
Since we can't have a fraction of a person, we need to round up to the nearest whole number. Therefore, we need 12 men to build a 200 m wall in four hours.