The book of 1 Peter was written in 60 A.D., 31 years after the death, burial and resurrection of Jesus Christ.
Peter was speaking present tense when he said 'them that are dead'.
He was speaking past tense when he said the Gospel was preached".
Your interpretation makes no sense whatsoever. In order for your interpretation to work, the verse would have to say:
1 Peter 4:6 (KJV)For for this cause is the gospel being preached also to them that are dead, that they might be judged according to men in the flesh, but live according to God in the spirit.
or
1 Peter 4:6 (KJV) For for this cause was the gospel preached also to them that were dead, that they might be judged according to men in the flesh, but live according to God in the spirit.
But the Word of God does not say that, does it? NO! The Apostle Peter points to the past, then to the present.
1 Peter 4:6 (KJV) For for this cause was the gospel preached also to them that are dead, that they might be judged according to men in the flesh, but live according to God in the spirit.