Diamond consists of a relatively simple lattice of carbon atoms wherein only a few impurities can substitute into the structure. Despite the apparent simplicity, the diversity of defects that can exist in the diamond lattice is astounding. Nitrogen is the most abundant impurity in diamond; one of the defects commonly seen in natural diamonds, the H4 defect, consists of four nitrogen atoms and two vacancies. It is strongly associated with B-aggregated nitrogen impurities, which consist of four nitrogen atoms surrounding a single vacancy. H4 is created through a fairly straightforward formation mechanism where a single vacancy is trapped adjacent to a B-aggregate of nitrogen. H4 is often seen in irradiated and annealed diamonds with suitable amounts of B-form nitrogen, and is a mature aggregate of nitrogen that is very difficult to achieve in synthetic diamond growth or even post-growth treatment.

https://www.gia.edu/gems-gemology/spring-2017-labnotes-h4-defect-synthetic-diamond