The force acting on the package of nails is the force of gravity. Here's how to calculate it:

* Force of gravity (weight) = mass x acceleration due to gravity

* mass (m) = 4.0 kg

* acceleration due to gravity (g) = 9.8 m/s²

* Force (F) = 4.0 kg * 9.8 m/s² = 39.2 N

Therefore, the force acting on the package of nails is 39.2 Newtons.