Because it was not identified, it is unclear whether Clinton realized she was potentially compromising classified information.
The IG reviewed a "limited sampling" of her emails and among those 40 reviewed found that "four contained classified [intelligence community] information," wrote the IG Charles McCullough in a letter to Congress.
McCullough noted that "none of the emails we reviewed had classification or dissemination markings" but that some "should have been handled as classified, appropriately marked, and transmitted via a secure network."
The four emails in question "were classified when they were sent and are classified now," spokeswoman Andrea Williams told CNN.
McCullough said that State Department Freedom of Information Act officials told the intelligence community IG that "there are potentially hundreds of classified emails within the approximately 30,000 provided by former Secretary Clinton."
Clinton knocks 'inaccuracies' in reports
Clinton on Friday criticized what she called "a lot of inaccuracies" in brief remarks about the classified emails.
She added that she had released 500 pages of emails and that "I've said repeatedly that I will answer questions" posed by a congressional panel.
"But I'm also going to stay focused on the issues, particularly the big issues that really matter to American families," she said.
In the past, Clinton has denied sending classified information from her personal sever.
"I did not email any classified material to anyone on my email," Clinton said at a news conference in March. "I'm certainly well aware of the classified requirements and did not send classified material."
The letter was first reported by The Wall Street Journal.
The Journal story broke just as the Justice Department issued a statement correcting earlier reports that the probe into email from Clinton's server during her time as secretary of state was a criminal investigation.
Instead, a Justice Department official said that the agency had "received a referral related to the potential compromise of classified information."
The New York Times first reported
late Thursday that inspectors general for the intelligence community and the State Department have asked the Justice Department to launch a criminal investigation into Clinton's possible mishandling of classified email. The Times significantly revised its story Friday to say the matter was referred to Justice to examine whether sensitive government information was mishandled in connection with Clinton's account -- but not necessarily by Clinton.
Sources confirmed both inspectors general have asked DOJ to open an investigation. The inspectors general are independent officials who conduct audits, investigations and inspections in the agencies for which they're responsible.
No decision yet to launch probe
There has not yet been a decision on whether to launch a criminal probe.
In response, Clinton campaign spokesman Nick Merrill issued a statement that "It is now more clear than ever that The New York Times report claiming there is a criminal inquiry sought in Hillary Clinton's use of email is false."
He added, "This incident shows the danger of relying on reckless, inaccurate leaks from partisan sources."
The request for the Justice Department investigation came after a June 29 memo from the State Department's inspector general stating a review of 55,000 pages of Clinton e-mails found "hundreds of potentially classified emails within the collection."
A follow-up memo from both the State Department and intelligence community inspectors general to Undersecretary of State Patrick Kennedy on July 17 said they had received confirmation that "several of the emails contained classified (Intelligence Community) information, though they were not marked as classified. At least one of the emails has been released to the public" by the State Department. Officials were additionally concerned that possible classified material would be posted in future releases of Clinton's emails.
Clinton's use of the personal account for official business instead of a State Department one stirred up a political storm around the 2016 Democratic presidential contender after news of it emerged in March.
Republicans criticize Clinton
The reported request for an inquiry is likely to renew fierce Republican criticism of Clinton's email practices.
Following the Times report this morning, GOP congressional leaders began launching attacks.
House Speaker John Boehner of Ohio released a statement calling on her to relinquish her server and questioning her judgment.
"If Secretary Clinton truly has nothing to hide, she can prove it by immediately turning over her server to the proper authorities and allowing them to examine the complete record," he said.
"What these reports demonstrate is the inherent risk of conducting our nation's diplomacy and foreign policy on your home email and personal server," he continued. "Her poor judgment has undermined our national security."
South Carolina Rep. Trey Gowdy, chairman of the Select Committee on Benghazi, echoed Boehner.
"The best -- the only way -- to resolve these important factual questions is for her to turn over her server to the proper authorities for independent forensic evaluation," he said in a statement.
The top Democrat on the Benghazi Committee, however, pushed back on the attacks.
"I spoke personally to the State Department Inspector General on Thursday, and he said he never asked the Justice Department to launch a criminal investigation of Secretary Clinton's email usage," said Maryland Rep. Elijah Cummings. "Instead, he told me the Intelligence Community IG notified the Justice Department and Congress that they identified classified information in a few emails that were part of the FOIA review, and that none of those emails had been previously marked as classified."
Kerry asked for recommendations from inspector
The State Department in May began publicly releasing some of the tens of thousands of emails
from the private account that Clinton handed over to it in December.
Officials from the department have been reviewing and redacting the huge collection of documents before putting them in the public domain.
Clinton has said she has nothing to hide and would like the process of making the emails public to be expedited.
Clinton's successor as secretary of state, John Kerry, wrote to the State Department inspector general in March asking for recommendations on the department's practices on document retention and its freedom of information process, a senior state department official told CNN.
"The Inspector General's assistance is welcomed as we take efforts to improve our policies with respect to the preservation of records in a digital environment," the official said.