you should have learned that the gravitational force (weight) between any two objects is inversely proportional to the square of the distance between their center of masses. translated into an equation ...
where W = the gravitational force , r = distance between them in earth radii, and k is a constant of proportionality.
now, you are given that when r = 1 earth radii , W = 800N ... find k.
Once you've found k ... when r = 4 earth radii, W = ?