It's going to be more trouble switching back and forth from 3-axis mode, but there's no inherent problem running a 4th (A) axis motor off the same driver as is used for the Y axis motor (as long as they're the same type of motor). You will need to figure out a way to fix the Y axis in position at the Y=0 point, though, since it will tend to freewheel if it's not held by the motor.
Just switch the Y axis leads to the A axis motor (assuming they're compatible) and reassign the Y axis to A. I'm not sure what software you're using, so I can't give you detailed instructions on how to do this, but if it's difficult you can alternatively just edit your G-code to substitute Y for A.
To generate the G-code, you can use DeskProto (it's free for the first month). Pick an "along X" 4th axis strategy, and it should work fine, even for relatively complex objects, since only the X, Z and A (formerly Y) axes are used.
ComputerSculpture.com — Home Page for Discount Hardware & Software