What happens to the mass discrepancy when four hydrogen nuclei form one helium nucleus?

Answer

It is converted into energy

The resulting helium nucleus has slightly less mass than the initial four protons; this mass deficit is converted into a staggering amount of energy, primarily gamma rays and neutrinos, according to $E=mc^2$.

What happens to the mass discrepancy when four hydrogen nuclei form one helium nucleus?
hydrogenenergystarFusionhelium