What mass conversion ratio defines the energy liberated during the fusion of hydrogen into helium in the Proton-Proton cycle?

Approximately $0.7$ percent of the initial mass is converted directly into energy.

The generation of stellar energy via nuclear fusion is a direct consequence of mass defect, quantified by $E=mc^2$. In the proton-proton chain, four hydrogen nuclei fuse to form one stable Helium-4 nucleus. Due to the binding energy required to hold the new nucleus together, the resulting Helium-4 nucleus possesses slightly less mass than the sum of the four initial hydrogen nuclei. This deficit, or mass difference, is the specific amount of matter that vanishes from the physical world and is instantly converted into vast amounts of radiant energy. For the Sun, this slight mass differential amounts to converting approximately 674 million tons of hydrogen into 670 million tons of helium every second, resulting in four million tons of mass becoming pure energy instantaneously.