The fourth week had twice the number of employees as the third week, so 400/2 = <<400/2=200>>200 employees on the third week.
In the third week, they hired 150 fewer employees than on the second week, that is 200-150 = <<200-150=50>>50 employees hired on the second week.
The first week had 200 more hirings than the second week, that is 50+200 = <<50+200=250>>250 employees.
The total number of employees hired is 250+50+200+400 = <<250+50+200+400=900>>900.
The average number of employees hired per week is 900/4 = <<900/4=225>>225 employees.
#### 225