© Alliance/DPAOleg Sentsov, accused of terrorism charges.
The FSB, the successor to the KGB, said in a statement that the four detained this month were "to be charged shortly" with "terrorism", organising a "terrorist" group and arms trafficking.
If convicted of terrorism, they face jail terms of up to 20 years.
The suspects are "providing confessions," the FSB said, accusing them of being members of the Right Sector ultra-nationalist paramilitary movement.
The FSB also accused them of plotting to down powerlines and destroy railway bridges as well as blow up a World War II memorial and a statue of Lenin.
The detainees include Ukrainian film director Oleg Sentsov, an opposition activist and member of protest group AutoMaidan, which held drive-by protests against president Viktor Yanukovych.
Ukraine reacted angrily to Russia's accusations against the detentions and called for their release.
"We insist on the release of Sentsov and the other Ukrainians detained in Crimea and we consider that the charges that are being pressed against them are absurd," Ukrainian foreign ministry spokesman Vasyl Zvarich told the Interfaxnews agency.
Sentsov was detained by Russian security forces on May 11 in Crimea, Russian television reported.
The FSB said it had searched the men's homes and found "explosive materials, guns, ammunition, canisters containing an incendiary mixture and builders' helmets" as well as gas masks and spraypaint.
The FSB said the detained had planned bomb attacks on the Eternal Flame war memorial and a Lenin statue in Crimea's main city of Simferopol on the eve of May 9's Victory Day holiday.
The group is also alleged to have plotted arson attacks on a community centre for ethnic Russians living in Crimea and the headquarters of ruling party United Russia in Simferopol.
Source: Agence-France Presse
Comment: It seems that Russia is keeping an eye out for attacks from the supporters of the new Ukrainian puppet-regime.