10:4 And when the seven peals of thunder had spoken, I was about to write; and I heard a voice from heaven saying, "Seal up the things which the seven peals of thunder have spoken, and do not write them."

And when the seven PEALS OF THUNDER had spoken, I was about to write;

John is just doing what he is told -- Revelation 1:19.

and I heard a voice from heaven saying, "SEAL UP the things which the seven PEALS OF THUNDER have spoken, and DO NOT WRITE THEM."

The voice is different from the loud voice (verse 3); but whose it is not clear.

Why John was not to write about the seven thunders will never be known to us. Some things either are too horrible or "unearthly" such that we are not able to understand. For example, science is just beginning, they think, to understand how matter is constructed. Yet a unified theory of physics has yet to be produced and tested. Perhaps, the utterances are related to physical things of which we have no knowledge.

One other person, Daniel, too, saw things he was to seal up until "end times", Daniel 12:4, 9-13.

Maybe there were/are no "words" to describe the sayings of the thunders, 2 Corinthians 12:3-4.

In a sinister twist: Even as demons recognized Jesus, Matthew 8:28-34, as the loud voice of the strong angel boomed out, these may have been seven satanic utterances. Certainly, in a beautiful place such as heaven, we wouldn't want to know or even care what the demons have to say. That the demons are in heaven -- they are not cast down to the earth until Revelation 12:9. This assumes Revelation events are given in mostly chronological order.