British police said the four - Moazzam Begg, Richard Belmar, Martin Mubanga and Feroz Abbasi - were set free on Wednesday after being questioned by members of the Metropolitan Police Service's anti-terrorist unit.
"Shortly before 9pm (2100 GMT) four men arrested under the Terrorism Act 2000 on 25 January were released without charge," a police spokesman said.
The four were returned to Britain after spending up to three years in custody at the US detention centre in Cuba.
They were arrested upon arrival on Tuesday under Section 41 of the Terrorism Act, which "refers to the alleged involvement in the commission, preparation or instigation of acts of terrorism".
A Pentagon spokesman said the US knew the detainees would be released from British custody if no charges could be brought against them, but believed they were still a "significant threat".
Pentagon spokesman Lieutenant-Commander Flex Plexico said: "We continue to believe that these individuals pose a
significant threat. British authorities have assured us
that they will take necessary steps to address the threat posed by these individuals."
He would provide no details on the steps British authorities agreed to take.
Upon their release, the four were escorted to locations of their choice to be reunited with their families.
Washington believes the four to be terrorists linked to al-Qaida or other groups, but London has argued that the US-run military tribunals for its Guantanamo detainees do not meet international standards sufficient to guarantee a free trial.
The four were all arrested in Afghanistan or Pakistan, or had previously visited one or both the countries.