1 The forum comments are under moderation.
2 The forum comments are in moderation.
3 The book is in revision.
4 The book is under revision.
Could 1 & 2 be the same? Could 3 & 4 be the same too? How are "under" and "in" different?
Under revision means that some one is assessing the content and may make some changes to it.
In revision means the same thing but it's more like being in the queue or the process of revision.
In moderation means within reasonable limits. So I wouldn't use it in your case. Under moderation on the other hand means that the comments are being moderated by a moderator.
I think in the first case the difference would be that "under moderation" would imply any comment posted will undergo moderation before being released, a general policy concerning all comments in the forum. "In moderation" would mean posts are currently locked up and being reviewed by moderator, a particular momentary state of specific posts. A bit like luggage "under surveillance", but "in customs control".
I don't think there's a difference between 3 and 4.