Garzon became internationally famous for his pursuits of Augusto Pinochet, the former Chilean president, and Argentine military officers, which helped set precedents for the principle that certain serious crimes can be prosecuted anywhere in the world.
He said that he would ask US authorities for copies of documents declassified by the current US administration, detailing practices such as waterboarding - which induces a sensation of drowning.
Earlier this month, President Barack Obama said that intelligence officials who used waterboarding and other harsh interrogation techniques on so-called terrorism suspects would not be prosecuted for their actions.
The documents, including several memos dating from the Bush-administration era, authorised the harsh interrogation of detainees by CIA employees.
Garzon said: "They reveal what had previously been suspected: an authorised, systematic plan for the torture and maltreatment of people deprived of their liberty without charge and without the most basic rights granted to any detainee."
Garzon detailed a list of abuses against the four men, including sexual assault, beatings, subjection to extreme heat and cold, continual interrogation at any time of day or night, sleep deprivation and long periods in handcuffs and shackles.
Human-rights groups in the US have called for US courts to investigate events at Guantanamo, and Obama last week left the door open to possible prosecutions of officials who laid the legal groundwork for torture.
Garzon's criminal investigation comes just weeks after he was forced to give up an attempt to initiate a probe into six former Bush administration officials, including Alberto Gonzales, the ex-attorney general, over alleged abuses at Guantanamo.
Spain's attorney-general said it was not legally allowed.
But one of the human-rights lawyers who had presented the criminal complaint against the six officials said that Garzon's latest ruling was far more than they had hoped for, as it went after not only the perpetrators of abuse but also their bosses.
Gonzalo Boye said: "We asked for a cup of tea and he has given us a complete breakfast."
In Guantanamo-related news, Eric Holder, the US attorney-general, said on Wednesday that Washington had identified about 30 inmates who are ready to be released and that the US would ask European allies to accept detainees within weeks.
|Holder said about 30 inmates at Guantanamo Bay are ready to be released [AP]
Holder was speaking in Berlin on the third stop of a European tour that has taken him to London and Prague.
"We expect to be reaching out to specific countries on specific individuals relatively soon," Holder said.
He said the requests would be made "within weeks as opposed to months".
Holder said the US was working to provide allies with detailed information on individual inmates and would ask individual countries to accept specific detainees when the time came, rather than making a blanket request.
Some European countries, including France and Portugal, have indicated a readiness to accept former Guantanamo inmates.
But Germany has declined to commit to such a move and Wolfgang Schaeuble, the country's interior minister, who met Holder on during his visit, has openly expressed reservations about taking in detainees.