How is the energy released during stellar nucleosynthesis, following Einstein's equation, fundamentally derived?

The mass of the resulting helium nucleus is slightly less than the original hydrogen nuclei

The colossal energy output characteristic of nuclear fusion stems from a direct conversion of mass into energy, precisely described by Einstein's mass-energy equivalence, E=mc². When four hydrogen nuclei fuse to create one helium nucleus, careful measurement reveals that the resulting helium nucleus possesses a minute fraction less mass than the combined mass of the four starting protons. This discrepancy, though seemingly small on an atomic scale, is liberated as immense energy because the conversion factor, the speed of light squared (c²), is enormous. This released energy manifests as the kinetic energy of the products and the high-energy photons that radiate as heat and light, distinguishing stellar fusion from less energetic chemical reactions which only rearrange electrons.