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.
Personally, I am not after rotary right now, but it may be something I look into in a year or so. Right now, I am trying to wrap my head around the vector software and cutting paths. Hopefully it will be a little clearer, when I have the machine in front of me and can do trial carves.
I just did a quick look on the inventables forum, and found a couple of threads. There are more.
To build a normal CNC router, you need to know every parts of it.
You can find most CNC router parts on : http://www.cncsparetools.com/cnc-router-spare-parts.html ,
also each parts with their detailed explaination, that helped me a lot when I built mine last year.
And you can find lots of information and knowledge about CNC machine parts, so you can choose the right one for your specific needs.
Good luck !
X-Carve can not support a 4th axis. The controller is GRBL on an Arduino, so it's limited to 3 by both hardware and software. When you see somebody with a 4th axis on an X-Carve they're replaced the controller. Alain Vaillancourt (The Woodpecker on YouTube) recently did that: https://www.youtube.com/user/lgosseuxdbois.