ha4

HA4 is an alphanumeric designation used in multiple, unrelated contexts. Its meaning depends on the field and local conventions.

In the United Kingdom postal system, HA4 denotes a postcode district within the HA (Harrow) postcode area in north‑west London. As a postal code it is used for sorting mail and as a geographical reference for addresses, local services and statistical reporting.

In virology, HA4 (often written H4) refers to the hemagglutinin subtype 4 of influenza A viruses. Hemagglutinins are surface glycoproteins involved in viral attachment and entry; H4 viruses are primarily associated with avian hosts and are monitored in surveillance and research because of their potential to cross species barriers.

The string HA4 also appears as a model number, product code, technical designation or classification in other domains such as consumer electronics, transportation and standards documentation. In such cases the specific meaning is determined by its industry or regulatory context. When encountering HA4, consult the surrounding context or authoritative sources to determine the intended reference.