Using a 4 axis machine, requires a good working knowledge of the CAD/CAM software that will be used. Rotary engraving requires wrapping.
On a small radius with small designs, such as a pen, and the shallow depth, will require the machine to be very accurate on all axis, including the rotary to be practically perfectly level to the Z axis.
Most of the less expensive machines don't have the best resolution and repeatability. Some of that is due to the rigidity of the machine, and some is caused by excessive backlash.