They enjoy their sins, but they will pay for them in the end. There are no greater miseries than false joys. The reference is to the Christians in these churches who have already died. But because they believed the gospel and lived according to it, because they put off their sins in Christ's name while they were alive in the world, we can be sure of their salvation and eventual resurrection to eternal life. To the casual observer it may seem that the gospel had no effect.
Christians die just like other people.
And from a human viewpoint, they may not even be popular in the world because of their lifestyle. But the gospel is a message of final victory over sin and death and of eternal life in a spiritual world, that is eternal and sinless world of glory. In fact, Peter's argument in the opening verses of chapter 4 is quite conventional. That is, it is very much like quite a number of other passages in the New Testament. Even the details are conventional.