According to the following scripture it states that judment was given to them, however it does not say that they were judged.
Rev 20:4 And I saw thrones, and they sat upon them, and judgment was given unto them: and I saw the souls of them that were beheaded for the witness of Jesus, and for the word of God, and which had not worshipped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands; and they lived and reigned with Christ a thousand years.
The following scripture should explain what is meant by judgment was given to them.
1 Cor 6:2 Do ye not know that the saints shall judge the world? and if the world shall be judged by you, are ye unworthy to judge the smallest matters?
1 Cor 6:3 Know ye not that we shall judge angels? how much more things that pertain to this life?
I hope that helps regarding judgment in Revelation 20:4
God bless you!