Secretary of State Hillary Clinton, during testimony Wednesday on Capitol Hill, heatedly told Wisconsin Sen. Ron Johnson:

“With all due respect, the fact is we had four dead Americans. Was it because of a protest or was it because of guys out for a walk one night decided to go kill some Americans? What difference, at this point, does it make?”

Actually, “we had four dead Americans” at the U.S. Consulate in Benghazi, Libya, on Sept. 11, 2012, neither “because of a protest” nor “because of guys out for a walk one night decided to kill some Americans.”

We had four dead Americans — including Ambassador Christopher Stevens — because of a terrorist attack.

And that tragic outcome might well have been averted if numerous warning signs about insufficient security at the Benghazi post had been heeded.

Thus, learning what went so terribly wrong before and during that attack is important.

So is learning why U.S. Ambassador to the United Nations Susan Rice, five days after that outrage, perpetuated the falsehood that the murderous assault on an American diplomatic installation was not a terrorist attack but a “spontaneous protest” of an obscure anti-Muslim video.

Reviewing the failures that led to those needless deaths could still make a crucial difference in State Department installations around the world.

Meanwhile, it’s essential to assign responsibility for any shortcomings in security for U.S. diplomatic personnel.

Secretary Clinton’s testimony Wednesday before the Senate Foreign Relations and House Foreign Affairs committees was much anticipated. Her scheduled appearance before Congress last month was postponed as she recovered from a concussion.

And Mrs. Clinton, who as planned long before Benghazi is stepping down as secretary of state with President Barack Obama entering his second term, did offer some positive points Wednesday.

She said that, under her orders, “all 29” of the recommendations from an Accountability and Review Board that examined the Benghazi attack will be “implemented quickly and completely” — and that she also has required “additional steps above and beyond those in the board’s report.”

Sen. John Kerry, President Obama’s nominee to replace Mrs. Clinton as secretary of state, has provided similar assurances.

Secretary Clinton also acknowledged again Wednesday that it was “a terrorist attack” in Benghazi, adding:

“As I have said many times since September 11, I take responsibility. Nobody is more committed to getting this right. I am determined to leave the State Department and our country safer, stronger, and more secure.”

However, Secretary Clinton testified that she was unaware of multiple appeals, from Amb. Stevens and his subordinates, for enhanced security at the Benghazi Consulate.

She explained: “These requests do not normally come to the secretary of state. They are handled by security professionals in the department.”

Then in this case, those “professionals” obviously should have handled those requests differently. And there should be severe professional consequences for their inaction.

That isn’t 20/20 hindsight.

It’s common sense: Islamic militants had repeatedly attacked the Benghazi compound, at one point last summer using an explosive device to blow a hole in its security gate.

Mrs. Clinton said Wednesday that she was making changes to assure that requests from U.S. diplomatic outposts for increased security will be given higher-level consideration.

She also correctly stressed that State Department staffers bravely accept inevitable peril when they take on international duty. As she put it:

“They represent the best traditions of a bold and generous nation. And they cannot work in bunkers and do their jobs. It is our responsibility to make sure they have the resources they need to do their jobs and to do everything we can to reduce the risks they face.”

But 4½ months ago in Benghazi, those risks were unacceptably — and avoidably — high. That did make a deadly difference for four Americans.

And correcting the mistakes that led to the Benghazi disaster can make a life-saving difference in the future.