The ex-hostages who had been held since 2007 were freed in apparently good health, while the Revolutionary Armed Forces of Colombia said one rebel was killed and another reported missing after the operation to release the captives.
"It was 20 difficult months," said policeman Juan Fernando Galicia after getting off the Brazil-provided helicopter that shipped him, two other policemen and a soldier to Villavicencio, 90 kilometers (56 miles) southeast of Bogota, around 7:00 pm (0000 GMT).
The four men -- part of a group of six hostages the Marxist rebels have promised to free by Wednesday -- were to travel to Bogota late Sunday for medical checkups.
Soldier William Dominguez appealed for the rescue of the remaining hostages, estimated to number between 350 and 700.
"We have to fight for all of the prisoners who are still in the jungle and together, we have to rescue them. We cannot leave them behind," said Dominguez.
The other two men released were policemen Alexis Torres and Jose Walter Lozano. They were all greeted at the airport terminal by Colombians for Peace, the group that helped broker the handover.
The fate of the ex-hostages had remained uncertain for several hours in the afternoon, after a member of the delegation told Venezuelan television of Colombian army operations in the area where the captives were released.
"Colombian army aircraft were an obstacle for several hours of the operation," delegation member Jorge Enrique Botero told Telesur television.
"There were continuous overflights ... for two hours," he said, accusing the military of failing to abide by a pledge to halt military activity during the handover and adding that they "almost made the rescue operation fail."
Jairo Martinez, a FARC commander, told Telesur that one guerrilla had been killed and another was reported missing and possibly captured near the area where the hostages were handed over.
The Colombian government denied the accusations as "unfounded."
The International Committee of the Red Cross (ICRC) had previously secured a commitment from the country's defense minister to halt military operations in Caqueta province, paving the way for the humanitarian mission.
The four hostages were received in the southern province of Caqueta by a delegation that included Senator Piedad Cordoba, representatives of the ICRC and members of Colombians for Peace.
Cordoba played a key role in talks to secure the hostages' release, a unilateral move that has marked an about-face for the rebels who had previously ruled out freeing captives without concessions.
On Monday, a similar operation was set to rescue Alan Jara, a former governor abducted in 2001. The rebels were expected to release on Wednesday former lawmaker Sigifredo Lopez, kidnapped in 2002.
Once all six hostages are released, the FARC rebels will only hold 22 "political" hostages. Jara and Lopez are the last politicians held by the group.
The captives are part of a group of so-called "political hostages" the FARC has wanted to swap for some 500 guerrillas held in Colombian and US jails.
In a statement posted online, the FARC had promised on December 21 to release the hostages.
The group has described the move as a "goodwill" gesture, while the Colombian government has called it a "maneuver" designed to ease military pressure and gain international sympathy.
The release reverses a FARC decision announced in early 2008 to stop handing over hostages without a demilitarization of certain regions.
The rebels last carried out a unilateral release of hostages in January and February 2008.
In July, French-Colombian Ingrid Betancourt -- a former presidential candidate -- was rescued in an operation mounted by Colombian spies who tricked the rebels into handing her over along with 14 other hostages.
Former Colombian lawmaker Oscar Tulio Lizcano managed to escape his captors in October.
The FARC -- Latin America's oldest and most powerful guerrilla force -- has been trying to topple the Colombian government since the 1960s.