For helical gears, a 4th axis cnc setup will also require some kind of a swivelling head attachment, to set the cutter parallel with the helix.
FYI, you can machine helical gears most easily on a universal horizontal milling machine (a manual mill). What sets this machine apart from regular horizontal mills, is that the table can swivel up to 45 degrees in either direction. However, a universal vertical head can also attain the correct angle, its just a matter of where the cutter is positioned: on top, or on the side of the blank being cut.
If you can find some pics I posted in my gallery on this forum, I show some action photos of the process all set up. You've gotta love gears to do these, though, and I'm talking about the gears that you use to connect the universal dividing head to the table. The table needs to drive the dividing head rotation, so it is very nice to have a power feed on that axis.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)