Platform: OS X Server · Yosemite 10.10+
Server 4.0 availability confirmed by Apple; requires Yosemite.
Apple source ↗Apple confirmed OS X Server 4.0 on October 16, 2014, available on the Mac App Store for US $19.99 and requiring Yosemite. The free client operating system did not automatically make the server add-on free. Version 4.0 belongs to the application; the underlying system is OS X 10.10.

Apple · Yosemite / Server 4.0 · 2014-10-16
In his November 2014 Ars Technica guide, Andrew Cunningham described a Yosemite-style interface, Internet reachability checks and an Access tab with network-based access rules. These are features described in his contemporary guide, not an independent security measurement or a guarantee that every service was reachable.
Andrew Cunningham · Ars Technica · 2014-11-17 · introduction/setup/Server.app basics
Apple's security documentation for Server 4.0 records fixes in Wiki Server, Xcode Server, mail and Profile Manager. It also describes disabling SSL 3.0 in Web Server, Calendar & Contacts Server and Remote Administration in response to attacks against that protocol. These are changes in that version, not a claim about present-day security.
Apple · OS X Server 4.0 · Security
Apple's archive dates Server 4.1 to April 8, 2015, and 4.1.5 to August 13, 2015. The table preserves only these verified milestones and the 4.0 release. Modern document revision dates are not treated as release dates, and the features Cunningham describes remain attributed to his guide.
Apple · Yosemite / Server 4.0 · 2014-10-16 · Andrew Cunningham · Ars Technica · 2014-11-17 · introduction/setup/Server.app basics · Apple · Security updates · 2015 archive
Selected releases, not a complete history. Dates follow Apple's release records, not the latest page edit. No builds are included.
ISO date: year-month-day.
Platform: OS X Server · Yosemite 10.10+
Server 4.0 availability confirmed by Apple; requires Yosemite.
Apple source ↗Platform: OS X Server · Yosemite 10.10+
Update date recorded by Apple; compatibility is version-specific.
Apple source ↗Platform: OS X Server · Yosemite 10.10.5+
Update date recorded by Apple; compatibility is version-specific.
Apple source ↗