Four axis is fairly simple; there is a rotary mechanism with an axis (normally) oriented parallel to the X axis of the machine. The 4th axis can be used just for positioning parts so they can be machined in different orientations as the pictures show. The first picture is parts that are slotted in one orientation and then drilled and counterbored in a position 90 degrees to the slot. Using a rotary axis allowed these operations to be done in a single fixturing. The second picture shows lengths of aluminum round bar that just gets machined to length and tapped in each end. Doing it this way on lengths that were cut off on a high speed bandsaw was much quicker than facing and tapping each end in a lathe.
The 4th axis can also be run under full program control so complex shapes can be sculpted.
the video of 4-axis cnc machine :
https://www.youtube.com/watch?v=ytp2tlz0c9o
