Clinton also said she directed the response to the attack from the State Department that night and "stayed in close contact with officials from across our government and the Libyan government."
In addition, Clinton said she immediately took steps to beef up security at U.S. posts around the world and to implement the review panel's 29 recommendations.
Clinton made clear that the security situation in North Africa and the Middle East remained threatening in the wake of the Arab Spring upheaval, with longtime leaders ousted in Egypt, Tunisia and Libya.
The fledgling Libyan leadership turned out to be unable to fulfill traditional security commitments to the U.S. diplomatic compound, she said.
"What I found with the Libyans was willingness but not capacity," she said.
Clinton also warned that weapons from Libya have turned up in Algeria and elsewhere, adding that "this Pandora's Box if you will" represented a major security threat.
"The Arab Spring has ushered in a time when al Qaeda is on the rise," she said. "The world in many ways is even more dangerous because we lack a central command [in al Qaeda] and have instead these nodes that are scattered throughout North Africa and other places."
Clinton expressed particular concern at events in Mali, where well-armed Tuareg militia, who had been working for former Libyan leader Moammar Gadhafi, came home just as al Qaeda in the Islamic Maghreb (AQIM) gravitated toward the area.
The size and topography of northern Mali, with its endless desert and caves, made for a long but necessary struggle, she said, adding that "we cannot permit northern Mali to become a safe haven."
Overall, she said, at least 20 U.S. diplomatic outposts "are under a serious threat environment as I speak to you."
Wednesday's committee appearances were some of the last acts for Clinton before she leaves her post as long planned, and Clinton showed a personal side in discussing what happened.
"For me, this is not just a matter of policy," she told the Senate panel. "It's personal."
In reference to the return of remains of the four slain Americans, Clinton said in a voice choked with emotion: "I stood next to President Obama as the Marines carried those flag-draped caskets off the plane at Andrews. I put my arms around the mothers and fathers, sisters and brothers, sons and daughters."
Democrats on both panels made a point of praising Clinton's service and noted that House Republicans have voted to cut funding for diplomatic security. However, Republicans rejected any connection between budget resources and vulnerability at the Benghazi compound, citing a report by a State Department financial officer.
The hearings provided Republicans with a final opportunity to question Clinton, considered a possible presidential contender in 2016, on camera before she leaves office. After the September attack, conservative Republicans focused on the issue to attack the Obama administration's handling of the Libyan revolution and the overall Arab Spring upheaval.
Several legislators made references on Wednesday to Clinton's possible political future, with Republican Steve Chabot of Ohio drawing a laugh from the secretary when he said: "I wish you the best in your future endeavors. Mostly."
Polls show strong public support for Clinton and her performance as secretary of state, with an ABC News/Washington Post survey released Wednesday showing 67% of respondents had a favorable impression of her.
Clinton was originally scheduled to testify last month but postponed her appearance as she was treated for illness, a concussion and a blood clot near her brain. The country's top diplomat returned to work just over two weeks ago.