The difference in mass between a helium-4 atom and an alpha particle can be attributed to the binding energy released during the formation of the alpha particle.
An alpha particle is essentially a helium-4 nucleus, consisting of two protons and two neutrons tightly bound together. When these particles come together to form an alpha particle, some of their individual mass is converted into binding energy according to Einstein's mass-energy equivalence principle (E=mc^2).
The binding energy is the energy required to separate the individual nucleons (protons and neutrons) in the alpha particle back into separate particles. This energy is negative, meaning that it is released when the particles combine.
Due to this released binding energy, the total mass of the alpha particle is slightly less than the combined mass of its constituent particles (protons and neutrons). This difference in mass is known as the mass defect or mass difference.
Therefore, the mass difference between a helium-4 atom and an alpha particle arises from the conversion of mass into binding energy when the constituent particles are bound together.