Let us consider the 4 kg and 2 kg mass as m₁ and m₂ respectively.

The forces acting on the masses m₁ are weight acting downwards which is m₁g; the tension on string acting upwards say T and the force on the string acting downwards which is m₁a .

Hence, m₁g - T = m₁a ------ Eqn (1) 

The forces acting on the masses m₂ are weight acting downwards which is m₂g; the tension on string acting upwards say T and the force on the string acting upwards which is m₂a .

Hence, T – m₂g = m₂a ------ Eqn (2)

Solving (1) and (2), by adding both equations, we can get

a = (m₁ – m₂)g / (m₁ + m₂) 

Also, Solving (1) and (2), by subtracting both equations, we can get

T = 2 m₁ m₂ g / (m₁ + m₂)

Referring to the diagram, the downward force on the pulley = 2T = 2 * 2 m₁m₂ g / (m₁ + m₂) = 4 (4) (2) (10)/ (4 + 2) = 160/3 = 53.33 which is approximately 54 N