https://www.twitch.tv/ is offering Twitch Prime Members: Sid Meier's Civilization IV: Complete Edition (PC Digital Download) for Free ('Click' on Prime Loot at the top of the page).
Note, Must log into your Twitch Prime Account or Start a Free Trial to redeem Game. The game will be available as a digital download via the Twitch Launcher.
The Complete Edition Includes:
Sid Meier's Civilization IV
Sid Meier's Civilization IV: Colonization
Sid Meier's Civilization IV: Beyond the Sword
Sid Meier's Civilization IV: Warlords