The game comes with Mods, and you can download mods too. They are safe and simple stuff, sometimes even simpler than dropping in a folder. They're sometimes self-installing! And the only compatability issues are if you have the right Civ4 patch (because sometimes Mods won't be updated to work with the latest patch right away) but when they are, it will be released either as a patch to the Mod, or downloading the whole Mod again, just with different files.
They will usually self-install, but if they don't just plop them in the Mods folder. There is no registry; all the files there are are just the files that the Mod uses to actually work (XML, Python, some art files of various formats such as DDS, and possibly DLL files), and also the .ini file that lets the Mod tell the game "I exist." Then just go into the game, click Advanced>Load a Mod>Click the Mod you want. OR an alternative would be to create a shortcut, the target is, in quotes, the path to the .exe file of the game (C:\Program Files\Firaxis Games...) followed by /mod= (yourmod). Make sure there is a space between the path to the .exe and the /, and one between = and (yourmod).
You can find these Mods all over the place. Check in the Creation and Customization forum, in its Modpacks subforum.