When an object is dropped from a height there is a force acting on it due to the gravitational attraction of the Earth that accelerates the object towards the ground.
This acceleration is independent of the mass of the object. The reason behind this is that the gravitational force of attraction between the the object and the Earth is given by F = G*Mo*Me/r^2. This force is also equal to the product of the mass of the object and its acceleration towards the Earth or F = Mo*g.
G*Mo*Me/r^2 = Mo/g
=> g = r^2/G*Me
All the terms on the right hand side of the equation given above are constants. This gives a constant value for g which at sea level is 9.8 m/s^2
As the ball weighing 300 g and the ball weighing 1 kg accelerate downwards at the same acceleration from a height of 4 m, their velocity when they reach the ground is the same.
Any two objects dropped from the same height have an equal velocity when they strike the ground irrespective of their mass.