True 4th Axis milling

While we're on the subject of 4 axes, how about a
discussion of true 4th axis milling. No, I don't mean
a substitution of the X or Y axis by the rotary axis,
but rather all four axes interpolated so that a circle
ends up as a circle, instead of stretched as it does
with substituted X or Y.
A friend of mine has MasterCam v.8 which is
supposed to write true 4 axis code. Trouble is, he
never uses that feature and doesn't really know how it
works.
Any list members know of a hobby level (price)
program that will write true 4 axis G-code? Or can
anybody explain that function in Mastercam v.8?
Bill Reidsema