The four’s release by pro-Russian rebels followed the exchange of 146 Ukrainian service personnel for 222 rebels on Friday.
The move was agreed as part of a 12-point peace plan in talks between envoys of Ukraine, Russia, the separatists and European security watchdog Organisation for Security and Co-operation in Europe (OSCE) on Wednesday.
“The head of the SBU (security service) reported the release of 146 Ukrainians to the president. They will all be able to celebrate New Year … with their families,” spokesman Svyatoslav Tsegolko said in a Facebook post before the swap took place.
Earlier, an SBU aide had said they would hand over 222 rebels for the Ukrainian servicemen.
The uprising by separatists began a month after Russia annexed the Black Sea peninsula of Crimea from Ukraine in March, following the popular overthrow of Ukraine’s Moscow-backed president.
The conflict has killed more than 4,700 people.
Kiev’s pro-Western government says Russia orchestrated the rebellion in Ukraine’s east, a charge denied by Moscow.
The peace protocol, agreed by Kiev and rebels in September, also includes a ceasefire.
Most of the plan has not been implemented due to repeated violations of the ceasefire and because separatists defied Kiev by holding leadership elections.
It is not known exactly how many prisoners are held by the two sides, but Ukraine’s military said this month about 600 Ukrainians were in rebel hands.
Roughly 1,300 people have been killed since the ceasefire was agreed in September, according to the United Nations, but the fighting lessened significantly this month.
On Friday, however, the military said rebels had slightly stepped up their attacks on Ukrainian positions in the east and reported that a Ukrainian servicemen had been killed during the past 24 hours.
“In the past two days, (rebel) fighters started using artillery and GRAD rocket launchers. Attacks have intensified to a minor extent,” Ukrainian military spokesman Andriy Lysenko told Reuters.
“Rebels are using the ceasefire to regroup their forces,” he said later in a televised briefing.
Authorities in Kiev had said there could be further peace talks between Ukraine, Russia, the separatists and the OSCE in Minsk on Friday, but Belarus’s foreign minister Dmitry Mironchik said that they would not take place, Belarussian Interfax reported, without elaborating.
Meanwhile, Ukraine’s suspension of transport connections with the Russia-annexed Crimean peninsula is expected to be only a temporary measure, a Ukrainian security official said yesterday.
Ukraine blocked rail and bus travel on Friday between the mainland and the peninsula.
Yesterday, the Crimean customs service said vehicles were also stopped at two of the three crossing points on the isthmus leading to Crimea, but traffic resumed in late afternoon.
Train and bus traffic was suspended because “there is a high likelihood of sabotage groups entering under the guise of local people,” Lysenko told a briefing.
He didn’t say how long the closure would last.
Ukrainian police were investigating three fatal explosions that took place in the south of the country.
Two of the explosions were in a city near Crimea and one in Odessa.
There have been strong tensions between pro-Russia residents and supporters of the Kiev government in Odessa, a major Black Sea port since more than 40 Russia supporters died when sheltering in a building set ablaze by firebombs in May.