A body weighs 10 N in the air. If it is immersed in water, it weighs 6 N. What will be the weight of the liquid displaced by the object?

According to the Archimedes principle: “When a body is fully or partially immersed in a fluid at rest, it appears to lose weight”. 

Here apparent weight, W' = W - F_b, where F_b = Buoyant force which is equal to the weight of the liquid displaced, W = weight of the body in air.
Apparent weight, W' = 6 N

                             6 N = 10 N - F_b

                              F_b  = 4 N 

                                  
Here, 4N is the buoyancy force or the weight of the liquid displaced by the object.