Although legal experts from Human Rights Watch (HRW) will observe the proceedings at Guantanamo Bay, Cuba, the group is concerned that "the military serves as prosecutor, judge, jury, appeals court and potentially even as executioner".
"The commission rules do not create a level playing field," said US advocacy director at HRW, Wendy Patten.
Authorised by presidential order in 2001, the commissions are set up to prosecute non-US citizens who have allegedly participated in international terrorist acts against the US. The commissions are staffed by current or retired members of the US military.
However, the normal rules of a court martial - a proceeding under which members of the US military are tried - do not apply in the commissions, said HRW.
For example, hearsay evidence is admissible and the defendant is not guaranteed the right to see all the evidence in the case, denying him the right to confront the evidence used against him.
Perhaps most critically, the rights group added, there is no appeal to a civilian court, as is the case with courts martial.
The commission will have power
of death sentence over detainees
"The military commissions offer no possibility for independent appeal, no matter how serious the error," said Patten.
Final review rests with either the US secretary of defence or the president, leaving the ultimate verdict and sentence entirely within the military chain of command.
While the four detainees to appear next week face a maximum sentence of life in prison, the commissions will have the power to sentence others to death.
Significant challenges already exist ahead of the first hearing scheduled for Tuesday.
One defence attorney has not seen his client in four months because of a government delay in giving clearance to a translator. Another lawyer has withdrawn from the case after accepting another job, leaving her client with no representation.
Others say the broad restrictions, which include the military's right to monitor conversations between attorneys and clients, make it nearly impossible to win their cases.
The four detainees are Ali Hamza Ahmad Sulayman al-Bahlul and Salim Ahmad Hamdan of Yemen, David Hicks of Australia and Ibrahim Ahmad Mahmud al-Qusi of Sudan.
The Geneva-based International Committee of the Red Cross has been the only independent group to have access to the prisoners. In October, it issued a public rebuke that called the prolonged detention "worrying".
Meanwhile, a US military review panel has decided against releasing 10 detainees in Guantanamo Bay, concluding they were properly classified as "enemy combatants", an official said.
Rights groups have campaigned
against the detentions
The decision brought to 14 the number of cases decided in hearings meant to evaluate whether each detainee was properly held as an "enemy combatant" or should be freed, said Pentagon spokeswoman Katy Wright. Not one of the 14 was released.
The review tribunals are separate from the military commissions, where the detainees are charged.
All of the 585 Guantanamo detainees are accused of links to the Taliban or al-Qaida, including a 30-year-old prisoner who allegedly served as Usama bin Ladin's bodyguard.
The military refused to release details of the concluded review cases, including when the detainees went before the panels, their names, nationalities and circumstances of their capture.
Human rights lawyers have criticised the hearings, saying they fail to satisfy a recent ruling by the US Supreme Court that prisoners may contest their detention in US civilian courts. Many have been held for nearly three years since the detentions began in January 2002.