Question: I have Yosemite. I am considering installing OS X server 4.0. According to [developer.apple.com](https://developer.apple.com/library/ios/documentation/IDEs/Conceptual/xcode_guide-continuous_integration/200-Adopting_a_Continuous_Integration_Workflow/adopt_continuous_integration.html#//apple_ref/doc/uid/TP40013292-CH3-SW1):

> 
> You can, however, install OS X Server and enable Xcode Server on your
>  development Mac.
> 
> 
> 

What exactly is OS X Server 4.0? Does installing require me to have a freshly formatted machine? In other words, does it overwrite Yosemite with a totally new OS (similar to Windows Server 2012 vs Windows 8)? Or, does it simply extend my Mac with new capabilities like a powerful program suite?

Answer: 
OS X Server 4 is an application. (.app) that is installed over the OS. It won't require you to format the entire drive to install it. It will just add the needed features to take advantage of the Server.app suite.
OS X Server is primarily an interface. OS X contains (almost - mail and databases are the main exceptions) all the software you will need, so if you want to set things up from the command line it won't make any difference. However, balance your time to work out the command line setup against the $20 Server.app costs.

Server.app is rather picky about configuration file layouts - if you want to do your own adjustments I recommend you make a separate file and use an include directive. Behaviour ranges from no problem to service won't start to Server.app crashing (so you can't reset it)