You don't say what "normal" 4-axis routine you're using, but it sounds like you've got your Y axis stationary and your tool is progressing down X, then the A axis increments in degrees and it comes back the other way. This is not going to be able to reach the eye sockets of that skull, or between David's legs. You need to use a different strategy: indexed or "N-sided" cutting. This would treat the skull like a 3-axis part, with the Y being free to move as well as X and Z. When you've cut half-way down the eye sockets, you rotate the part 90 degrees (in this case, "N"=4) on the A axis and cut from the side, then another 90 to cut the bottom, and so on. I'm not sure if Mastercam has a pre-planned routine like that, but you should be able to make it work. Make sure that Y and Z are zeroed to the center of the rotary axis so nothing shifts when you rotate the part. If you can't get Mastercam to do it before it crashes, try DeskProto,