The incident in December 2020 took place after a teacher wrongly accused the 15-year-old schoolgirl of possessing cannabis.
No other adult was present, her parents were not contacted, and no drugs were found.
The police watchdog said it received a voluntary referral from the Met on May 6 2021.
The Independent Office of Police Conduct (IOPC) said it began its investigation five days later, before reallocating the case in March 2022 to a new lead investigator who reviewed the evidence and previous decisions.
Following the assesssment, it decided to “escalate its investigation from one of misconduct to gross misconduct”.
A spokesperson said: “We can confirm that four Metropolitan Police Service officers have been served with gross misconduct notices by us in connection with our ongoing investigation into complaints that Child Q was inappropriately strip-searched.
“As with all of our investigations we continually review the evidence and lines of inquiry as the investigation progresses. As a part of this, matters were identified which required new notices of investigation to be served on officers.”
The officers were being investigated for “potential breaches of the police standards of professional behaviour at the level of gross misconduct, which does not necessarily mean that disciplinary proceedings will follow”, the IOPC said.
“We are also considering whether the child’s ethnicity played a part in the officers’ decision to strip-search her,” the spokesperson added.
When asked why it took several months between the incident occurring and the Met Police referring itself to the IOPC, the force said: “Information was provided to the child’s family to support any complaint they wish to make against the Metropolitan Police Service.
“A complaint was subsequently received and was referred to the Independent Office for Police Conduct in May 2021 for investigation.”
If gross misconduct is proved, the officers face dismissal. Their fate will be decided at a hearing in coming months.
At a special scrutiny meeting into the scandal at Hackney Town Hall on Monday evening, Hackney’s borough commander Det Ch Supt Marcus Barnett told councillors: “It is beyond regrettable that it ever happened to a young child.”
Councillor Anntoinette Bramble, Hackney’s deputy mayor with responsibility for education, young people and children’s social care, said: “An indefensible, inconceivable thing happened. In 2022, we are still judged by the colour of our skins.”
Local MP Diane Abbott said: “I am glad the IOPC has escalated the investigation, but the community is very concerned about how long the investigation is taking.”
The incident sparked days of protest in Hackney, where the incident took place.
The search of pupil, known as Child Q, was unjustified and racism was “likely” to have been a factor, a safeguarding report found.