What is h4 in chemistry?

H4 is not a molecule or compound in chemistry. It is just a notation or prefix used to denote the third ionization energy for certain elements such as Li, Be, B, C, N, O, F, and Ne. The third ionization energy is the energy required to remove the third outermost electron from an atom. It is usually much higher than the first and second ionization energies due to the increased stability of the resulting ion. The notation H4 is used because it is equivalent to the energy required to remove the fourth electron from a hydrogen atom.