According to the FARC, its three members and one family member were kidnapped, tortured and murdered in La Florida, a rural community in the southwestern Nariño province.
The alleged massacre was not immediately confirmed by the United Nations, which is in Colombia to monitor the peace process and investigate violence against the FARC.
The former guerrillas called on the ELN to respond to the allegations its members were responsible for the killing.
The FARC called on the government and the last-standing guerrilla group to resume peace negotiations and a ceasefire that expired last month.
The killing of the members of the FARC followed a month in which 21 social leaders were murdered, according to think tank Indepaz.
Nariño has been particularly violent. The ELN, dissident FARC guerrillas, paramilitary groups and drug traffickers are fighting over control over the vast coca plantations in the province.
Colombia’s state authorities have been criticized by the UN for failing to assume control over the territories abandoned by the FARC.
The ELN resumed attacks on oil infrastructure and security forces last month, blaming the state of the violence that has terrorized many communities in the countryside.
When the FARC was still in arms, its fronts in Nariño regularly clashed with ELN rebels over control of criminal rackets that funded both organizations.
Following its demobilization, the FARC has embarked on an election campaign ahead of congressional elections in March and the first round of a presidential election in May.
The group has urged improved security to avoid the repetition of an extermination campaign that virtually wiped out the communist Patriotic Union party after peace talks in 1985.
The ongoing peace process seeks to end guerrilla violence that has plagued Colombia since the 1960s.