Sullla, a well-known member of the Civilization community who worked as a beta tester for Civilization IV before its release, has compiled his testing reports from that time,together with a lot of screenshots and other information, and published them on his website: https://www.sullla.com/Civ4PR/civ4PR.html

This has been his way to celebrate Civ4's 20th anniversary. How time flies!

I've been reading them and they offer a very interesting insight about the development process of Civ4 and the many changes that the game underwent to become the gem we all know and love. If you are interested in game design, or just want to learn more about the history of the Civilization series, I highly recommend these reports.