I don't plan to make a full previewer type thing, although in theory it will be possible in the custom layer sometime in the future, but I would think about building in a syntax highlighter type thing for markdown so that it still shows the markdown characters, but bolds them, italicizes them etc where it can. However none of this will be happening any time soon if it ever does.
Sorry I was not enough clear at my first post. My thoughts went mostly for markdown syntax highlighting rather than some sort of render preview (far of the main purpose of 4coder I guess).