https://www.pcgamer.com/civilization-4-is-free-for-twitch-prime-subscribers-right-now/

Civilization 4 marked the start of a new generation of Civ games. It was the first in the series with a 3D engine, the first built from the ground-up as a multiplayer game, and the first with accessible modding. As game designer Soren Johnson told Fraser in November, he wanted to "start from scratch and re-evaluate everything" about the series up to that point. It's still worth playing today, and Twitch Prime subscribers can now do just that for free for a limited time.

All you have to do is head over to the game's Twitch page (or click the 'Prime Loot' icon next to your user name) to get your free copy—you'll need to download the Twitch desktop app to actually play it. You can do that here.