The "unattended bag" was discovered at a check-in counter for El Al Israel Airlines.
Armed police rushed to the scene, closing some of the area and evacuating them.
All flights from the terminal were temporarily delayed until further notice.
The Metropolitan police confiscated that they were investigating and a cordon had been set up.
A spokesman for Heathrow Airport later confirmed that an unattended bag had been discovered, which was now identified and the matter resolved.
He said, "Our teams responded last night to an unattended bag at Terminal 4.
"As a precaution, a very small portion of the check-in was temporarily suspended.
"The rest of the terminal remained open and fully functional.
"The bag was identified and the matter resolved.
"We apologize to all passengers who are harassed by it."
Terminal 4, at 105,481 square meters, is the second largest terminal in Britain's largest airport.
According to the Heathrow website, 9.5 million passengers were served on 48,137 flights last year.