I realize there's a sense of making things simpler, but I'm considering using JSPWiki to write up my Ph.D. thesis (yes, I know I am crazy, this has been demonstrated before), and I'd like to have at least one more level of headings, basically <h4> would do.

I'm even willing to tackle the coding for this, if nobody had an issue with it. I've posted a potential patch to allow IdeaReverseHeadings, which would work also with this — perhaps better...