Each of the four customers received delivery of 400/4= <<400/4=100>>100 cartons of milk.
Since each customer returned 60 cartons that were damaged during delivery, the number of cartons accepted is 100-60 = <<100-60=40>>40 cartons by each customer.
If each customer accepted 40 cartons, the total number of cartons that the four customers did not return is 40*4 = <<40*4=160>>160
#### 160