The three soldiers and a naval cadet were released into the care of the International Committee of the Red Cross (ICRC).
They were to be taken to the government-held town of Vavuniya for debriefing and medical checks.
Eleven other service personnel remain in captivity. All have been held since 1993 or 1994.
No explanation has been given for the releases, which came at the end of a visit to the men by their relatives arranged through the Red Cross.
The men had gone on hunger strike for two weeks earlier this month to draw attention to their plight, and demand government action to secure their freedom.
Negotiating tactic
The BBC's correspondent in Colombo, Susannah Price, says the Tamil Tigers' decision to free the four military personnel will put new pressure on the government to negotiate the release of the remaining 11 captives.
She says the Tamil Tigers are presumably looking to secure the release of suspected rebels currently in government custody.
The Association of Relatives of Servicemen Missing in Action said they were very happy about the release and would like to see a government representative meet the Tigers to discuss the next move.
However, the defence ministry said they had not been officially informed by the Tigers about any demands.
'Take no prisoners'
Meanwhile, the United States' Country Report on Human Rights Practices in Sri Lanka has said both sides in the continuing conflict appear to be killing enemy fighters rather than taking them prisoner.
It said that observers believe, on the government side, there is an unwritten "take no prisoners" leading to many Tiger fighters who could have been captured being killed instead.
The report said the Tigers admitted they kill security forces personnel rather than take them captive.
Related to this story:
Sri Lanka peace meeting hits snag
(21 Feb 00 | South Asia)
Fighting overshadows peace mission
(15 Feb 00 | South Asia)
What chance peace in Sri Lanka?
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Fighting escalates in Sri Lanka
(01 Jan 00 | South Asia)
Kumaratunga promises end to hatred
(22 Dec 99 | South Asia)
Analysis: Fifteen years of bloodshed
(18 Dec 99 | South Asia)
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