On Yom Kippur, the High Priest entered the Holy of Holies four times during the traditional Temple service.
The Holy of Holies was the innermost chamber of the Temple in Jerusalem and was regarded as the place of God’s most concentrated presence. Under the biblical and later rabbinic ritual system, it was not an ordinary destination for priests: the High Priest entered it only on Yom Kippur, the Day of Atonement.
The first entrance was associated with carrying incense and creating a cloud of smoke. He then entered with the blood of a bull offered for the High Priest and his household, followed by an entrance with the blood of the goat offered for the people. His fourth entrance was to retrieve the censer and complete the incense procedure.
The number four refers to separate entries into the chamber, not to every action performed inside it. The High Priest also entered other parts of the sanctuary, changed garments several times, immersed in a ritual bath five times, and sprinkled sacrificial blood repeatedly. The count comes from the detailed Temple-service tradition preserved in Mishnah Yoma and the Talmud.