Lift them both to a 10 ft elevation and the 4 lb spear will have 40 ft lb of potential energy and the 8 lb spear will have 80 ft lb of potential energy.
As they fall down, the potential energy is converted into kinetic energy. At elevation zero, the potential energy is zero and the 4 lb spear is traveling at a velocity that gives it 40 ft-lb of kinetic energy and the 8 lb spear is traveling at a velocity that gives it 80 ft-lb of kinetic energy.
To give the 4 lb spear 80 ft-lb of potential energy, you have to raise it 20 ft in the air.
When both spears have the same energy which one will penetrate the deepest? The light or heavier or will they penetrate the same?