Relations between Russia and Georgia have been further strained in recent months, since Russia stepped up support for Abkhazia and sent more troops into the region. Tbilisi has accused the Kremlin of plans to annex the territory.
"The four detainees were freed after nine hours of questioning," police said. "They were given back their automatic weapons and passports. The military cargo was confiscated."
Georgia's Interior Ministry said on Tuesday that the peacekeepers were transporting 35 crates of munitions, including guided missiles and anti-tank mines.
"This type of arms is not allowed in the conflict zone. This is a violation of existing agreements," police said on Wednesday.
Mamuka Kurashvili, a senior Defense Ministry official, said: "The Russians made an attempt to create a kind of military base at their headquarters in the Georgian district of Zugdidi, but we will not allow this to happen."
Georgia's Interior Ministry said police were continuing the investigation.
Russia's Defense Ministry said on Tuesday the arrest was "in violation of all regulatory norms in the buffer zone."
Russia has reacted angrily to arrests of its military personnel in the former Soviet republic. Georgia's brief detention of Russian officers on spying charges in 2006 resulted in a postal and transport blockade that was only lifted this year.
Moscow has denied Tbilisi's accusations of seeking to annex Abkhazia and condemned Georgia's policy toward the restive region as conducive to new bloodshed.
Georgia under the Western-leaning President Mikheil Saakashvili has sought to join NATO, the plans that have fueled tensions in relations with Russia.