I have Yosemite. I am considering installing OS X server 4.0. According to developer.apple.com:
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?