I have just (nearly) completed a 4th axis for my Harrison milling machine. It is based around one of the Harmonic Drive FHA actuators.
This is quite a lazy way to do it, they are pretty much a complete axis out of the box.
Rather than describe it all in detail here, here is a link to a blog post on the subject.
Bodgesoc Blogsoc: Harmonic Drive 4th Axis