Alston added that 'two of these individuals are full-time government employees, one has previously acted on behalf of the government, and the basis on which the fourth was identified and selected as an expert remains unclear'.
Human rights organisations and unions say executions in Sri Lanka have become a worrying problem, and various groups are targetted, including journalists critical of the government.
Sri Lanka has previously rejected calls by UN officials and rights activists to allow for independent investigations.
Sri Lanka announced victory over the Tamil Tiger rebels in May. In the fighting leading up to the defeat, the government refused to let journalists and other researchers into the conflict zone.
The Sri Lankan authorities still prevent independent journalists from entering the camps where some 250,000 Tamil refugees are being held.
International Committee of the Red Cross officials have termed the areas 'internment camps'. UN Secretary General Ban Ki-moon has called for the refugees to be allowed free movement and recently said he was concerned for their fate.
The government says it is screening the camps for suspected militants.