I see two main references to the "dead" in the Bible. The first are the physically dead, King Solomon talks about them in Ecclesiastes 9:5. In that verse he tells us that the "dead know nothing". This confirms what we already know, that the physically dead cannot benefit from teaching or preaching. They are dead.
The second are the spiritually dead. But I believe this verse is talking about the physically dead. Why physically dead?
If you go back to Chapter 3 verse 18, you see: "He was put to death in the body but made alive in the Spirit." The verse is talking about Christs sacrifice. If you apply that to this verse you see that there are followers of Christ that will lose their life ("judged according to men in the flesh") but be alive in the spirit. He goes on to talk about suffering for Christ, this would include being martyred as many early Christians were.
If you look at chapter 3 and 4 as a whole you see that he is talking about turning away from an old lifestyle, living for God, and preparing for the judgement of men that will come as a result of that. This judgement could include the loss of your life, which is still true in some parts of the world today.