The Iraq Islamic Party described Tom Fox, an American, Norman Kember, a Briton, and James Loney and Harmeet Singh Sooden, both Canadians, as "activists in a movement opposing the war in Iraq".
The statement released on Saturday said: "The party calls on the captors to free them because their kidnapping is a dream opportunity for the supporters of the war against our country who say that Iraqis cannot tell the difference between those who support them and those who oppose them."
The Committee for Muslim Scholars, a powerful Sunni organisation, has also called for the release of the hostages.
Meanwhile, the Muslim Association of Britain (MAB) sent one of its leaders, Anas Altikriti, of Iraqi origin, to meet Sunni figures and to seek the release of the British hostage.
The four were kidnapped on 26 November in Baghdad.
In a message broadcast by Aljazeera on Friday, their captors threatened to kill them by 8 December unless all detainees are released.
A deadline set by the kidnappers of a German woman in Iraq for the government in Berlin to meet their demands has passed, German reports said on Saturday.
Susanne Osthoff, 43, an archaeologist and a convert to Islam who has lived in Iraq for 10 years, was seized together with her driver on 25 November in the Ninava region of northwest Iraq.
Osthoff, seized last month,
had lived in Iraq for 10 years
A video released by her captors to German television and broadcast on Tuesday purportedly showed her and the driver blindfolded and surrounded by armed men.
According to the magazines Focus and Der Spiegel, the kidnappers set a deadline of three days for their demand to be met. This was that Germany should stop training Iraqi police officers.
The Foreign Ministry in Berlin said on Saturday that it was continuing its "intensive" efforts to free Osthoff.