Four hydrogen nuclei have a total mass of 6.693 x 10-27 kg.
They fuse into a helium nucleus of mass 6.645 x 10-27 kg.
The mass lost is 0.048 x 10-27 kg. Thus, only the fraction,
0.048/6.693 = 0.00717 of the original hydrogen mass was converted to
energy. In other words, when you start with 1 kg of hydrogen, a mass
of 0.00717 kg will be converted to energy during the fusion. This gives
an energy output of
E = mc2 = 0.00717 x (3.0 x 108)2 = 6.45 x 1014 Joules.
It is still a huge amount of energy, even though most of the kilogram
is still there afterwards (now in the form of helium).