A court in Minsk on September 16 found the four activists guilty of organizing an unsanctioned public gathering.
The men were detained on September 14 in front of a detention center in Minsk, where they had intended to hold a memorial service for Ihar Ptsichkin, who died in the center.
Prsichkin's sister, Iryna Miranovich, was also arrested during the attempt to commemorate Ptsichkin.
The court on September 16 warned her of possible consequences if she tries to organize an unsanctioned public event in the future.
Ptsichkin died in August while serving a three-month sentence for driving without a licence.
Relatives reject official statements saying Ptsichkin died of heart failure.
They say his body bore traces of beating.