So why were there 119 screws if it was fine to be missing 4 in a row? Aircraft are usually built with a factor of safety of 1.5-2 or so - meaning it's probably

So why were there 119 screws if it was fine to be missing 4 in a row? Aircraft are usually built with a factor of safety of 1.5-2 or so - meaning it's probably only in the design spec for one screw to be missing in a row...