On 15 June 2022, LIGO announced to start the O4 observing run in March 2023. As the date got closer, engineering challenges delayed the observing run to May 2023. An engineering run to assess the sensitivity of LIGO, Virgo, and KAGRA began in April, with the Hanford detector's first operations beginning on April 29, and the Livingston and Virgo detectors' first operations beginning on May 5.