Unita says the mercenaries were flying in a Pilatus 7 aircraft which they shot down in Lunda Norte, the northeastern province where five days before government forces, backed by up to 100 mercenaries, had driven Unita troops from the diamond producing areas of Cafunfo and Catoca.
Ironically, the fate of the four men, who like the majority of the other mercenaries once fought for the security forces of South Africa's white minority government, could depend on the diplomatic skills of the man once considered Pretoria's number one enemy: Nelson Mandela.
Earlier this month, Mr Mandela began a mediation effort to end Angola's 19-year-old civil war by hosting a meeting in Pretoria of Angolan President Jose Eduardo dos Santos, Zairean President Mobuto sese Seko - Mr Savimbi's main backer - and Mozambican President Joaquim Chissano, who originally proposed Mr Mandela's involvement. Mr Mandela is expected to meet Mr Savimbi later this week.
'We would like to stop this execution in the context of the intervention of the President (Mandela) was requested to make in the Angolan issue,' Deputy President Thabo Mbeki told reporters at the weekend. 'But we would not want to deal with this in a piecemeal way. The problem is greater than four mercenaries. The estimations are that there are something in the order of 500 of these mercenaries in Angola. Those people ought not to be there.'
Many of the mercenaries previously served with 32 Battalion, a notorious unit of Angolans, South African officers and foreign non- commissioned officers, who routinely fought on behalf of Mr Savimbi's National Union for the Total Independence of Angola (Unita).
As the South African Defence Force began to withdraw from Angola in 1988 and its covert operations units were disbanded, unemployment forced the soldiers to look elsewhere. Salaries of between pounds 1,300 and pounds 6,300 a month have attracted them back to Angola, this time on the side of the ruling Popular Movement for the Liberation of Angola (MPLA). At least four of South Africa's best helicopter pilots, and up to ten Israelis, are flying in Angola.
The main South African company recruiting former South African and Namibian soldiers for work in Angola is Executive Outcomes, a Pretoria-based security firm.
Executive Outcomes has confirmed that it employed two of the four men captured by Unita, including Daniel Scheurkogel, a former staff sergeant in the South African Defence Force.
Executive Outcomes officials have repeatedly maintained that their employees were not directly involved in fighting but were in Angola to train the government army, a claim dismissed by Western military analysts in Luanda and by Angolan officials. One senior Angolan official said up to 100 mercenaries fought alongside elite Angolan commandos in the April capture of the provincial capital of Ndalatando.
At least 11 South African mercenaries have either died or disappeared in Angola in the past 18 months. The first known casualty was Phillip Smith, a Briton resident in South Africa, who was killed early last year when Unita captured the northwestern oil-producing town of Soyo.