Not just martyred by the beast in the end time, but martyred through out human history. I suspect that it's only for the martyred because it seems to say that, and it keeps the Great White Throne Judgment following the 1000 year reign in Revelation 20:12 consistent with Matthew 25:32 where both the righteous and the unrighteous stand before the Lord in Judgment (popular teaching says the righteous are not present at the Great White Throne Judgment). But then that muddies up the timing and nature of the official second coming of Christ. Lot's to think about here. That's why I'm not dogmatic about this subject.
They are not martyrs in the sense of being killed for one's belief system falsely accused by another human belief system. Those in Revelation 20:4 were beheaded instead of recieving the mark. This is the only choice to make to actually remain in the Lamb's book of life. Stephen and all martyrs since would have died and gone to heaven whether they were martyred or died naturally of old age. Being martyrs did not give them salvation. When it comes to receiving the mark, it is a choice to remove one's name from the Lamb's book of life. To avoid the mark, one must be beheaded.
I think the sheep are part of the Millennium and are also resurrected, but they did not die because of natural causes nor for their faith. They were directly chosen by the Lamb to live per Matthew 25:31-33. They are resurrected the same way, but they were already judged at the judgment of Christ. The same with the wheat and the tares. These were not mentioned directly by John, because John does not even address the sheep and goats nor the wheat and tares. Jesus covered that in the Olivet Discourse. The church already left at the 6th Seal, the actual Second Coming event. Christ has to come to harvest the sheep and wheat, and destroy the goats and the tares. John just explains how the rest of humanity deals the the Trumpets and Thunders. Jesus Christ dealt with the harvest itself.
The only other group that continue on with Christ on earth during the Millennium are the 144k that were sealed between the 6th and 7th Seals. They are the firstfruit disciples who help in the harvest and rule with Christ over the harvest during the Millennium.
The church and all OT martyrs and NT martyrs are mentioned in the 5th Seal. They are glorified immediately before those alive and remain are raptured from earth in the 6th Seal at the Second Coming. Paul describes that event. The church will forever be with the Lord God in Paradise. Only after the NHNE will Paradise come down as the New Jerusalem. But a city that is 1200 miles high is practically in heaven at the top. That reality will be totally different than now.