The release of the four, all members of the Sea Tigers, the rebels' naval wing, came as a delegation of top rebels transited through Colombo on Friday en route to the talks in Geneva.
The meetings, begining on 22 February, will be the first direct talks in nearly three years between the Tigers and the Sri Lankan government.
The Geneva talks come as a Norwegian-brokered ceasefire has come under huge strain because of escalating violence.
The Tigers accuse the government of backing a breakaway faction and want officials to disarm that group - an issue that is expected to dominate the upcoming talks.
The government denies backing the splinter group and in turn blames the rebels for many of the deaths of more than 150 people, including 81 soldiers, since 4 December when violence flared up in the northeast.
The rebel movement split in 2004 when an eastern-based military commander named Karuna broke away with some 6000 fighters.
The rebellion was suppressed by the mainstream rebels; but Karuna and several other leaders managed to escape and are known to operate in eastern Sri Lanka.
The Tamil Tigers launched a violent campaign in 1983 to create a separate state for ethnic minority Tamils in the northeast, accusing the majority Sinhalese of discrimination.
The civil war killed 65,000 people before the ceasefire was signed in 2002.
Peace talks broke down a year later over rebel demands for wide autonomy.