A body of mass 1 kg is moving at 4 m/s and it collides with a stationary body with a mass of 750 g or 0.75 kg. After collision, the first body comes to a stop and the second moves at 5 m/s.
If friction could be ignored here, the law of conservation of kinetic energy would ensure that total kinetic energy is conserved. As the total initial kinetic energy is 1*4^2 = 16 J, the total final kinetic energy would be 16 J too. But the total final kinetic energy is 0.75*5^2 = 0.75*25 = 18.75 J. It is seen that due to the collision the total kinetic energy in the system has increased by 2.75 J.
This is not possible in an isolated system.
There is an error in the information provided by you.