When I first read about the enneagram (probably fifteen or twenty years ago), I mistyped myself as a Four. I could easily relate to the self-image of a brooding artist ready to burst with pent-up creativity. Over the years I've made a long mistyping journey through types Nine, One, and finally Six (which is still pretty new to me).
My main creative expression is via writing. I can't not write. If I'm not doing it in a forum like this, I'm doing it in a blog or a journal or somewhere else. Why do I do it? It's hard to say. If I knew, I might solve the enigma of myself and be finished, and then I wouldn't need to write anymore.
I can guess, though. I suspect it's part of a self-discovery process. The Fours I've met may also be going through that process, but they always seem to have some particular wound or inner mystery that they're compelled to build up emotion around; and insofar as they're creative, they'll express that in some artistic way. In their case, it seems to me it almost always comes out as something deeply personal; sometimes it's so unique that others may have a hard time relating to it even if they can easily appreciate the creativity behind it.
In my case, as a Six, I'm always aiming for self-certainty. I react with strong feelings to many things, and my own reaction intrigues me. So I write about it in hopes that I'll end up revealing to myself (and incidentally to others too) where I stand--which is part of who I am. Besides that, I'm a romantic; I feel a strong attraction to the object of my affection, and that's always a fitting subject for art or poetry or music (in my case, poetry is the most likely). Sixes have something of the desire to merge with another as Nines do, so the expression of love--the desire for a powerful romantic connection--might figure into Six-type creativity. (Fours, perhaps, would be more likely to express the longing sensations of unrequited love, or the bluesy gloom of a love gone bad.)
Often, though, especially as I've gotten older, I find myself turning my creativity to more prosaic kinds of self-expression: nonfiction, letters to friends, blog posts, and such. And often what I create begins as a reaction to something someone else (maybe a Four) has produced--something that moved me.
At times my creative muse speaks in the name of anxiety and/or loyalty: I'll respond to some political situation with vehemence, or I'll lavishly praise a noble spiritual cause. Perhaps Julia Ward Howe (writer/composer of "The Battle Hymn of the Republic") was a Six. A rousing anthem is just the thing a Six might be moved to create. In fact, some of the songs that I find especially moving have that quality to them: e.g., folk-rock protest songs of the 1960s.
So, yeah--as another post above says, the creative expression of Sixes often ties in with social movements or things that involve groups of people (even the whole human race), whereas the creativity of Fours is typically much more individual (even to the point of be indecipherable by anyone else).
Just some rambling thoughts on this interesting subject.