How to buy and/or download OS X Server 4 I am upgrading an older server. It was running Mavericks (10.9) and just upgraded to Yosemite (10.10). However, when I try to upgrade the Server app, the only option I have is the new macOS Server, which won't install on my OS. 

I've searched the web for hours looking for a legitimate way to buy and/or download this software, but there appears to be no way to do it. 

As I never bought the app, it doesn't appear in my Purchases list in the App Store.

Does any one know how I can get OS X Server 4?
I would suggest trying to purchase Server 3.2.2 if you did not already have that since running Mavericks. Then look under purchases and if you can see the option to install macOS Server do that. On Yosemite it will ask you to install the latest compatible version Server 5.0.15. (I just installed Yosemite 10.10.5 and Server 5.0.15 on it). Server 4 however seems gone forever.

Reference:
https://support.apple.com/en-us/HT203137