Answer
6 N, downward. Zero newtons.
Work Step by Step
The 10-N force due to gravity (also known as the weight, of course) points downward, and if the object is falling, the 4-N force due to air resistance points upward. The net force is 6 N, downward.
If the force due to air resistance is 10 N, it cancels the weight and the net force is zero.
This is discussed on pages 65-66.
