The alpha has always been moddable.  It works the same way it did in GalCiv3.  The full game is supposed to have something similar to the workshop in Steam so that mods can be used no matter where the game was purchased.

Editing the xml files where they exist doesn't work.  A folder for a mod must be created in the Mods folder, the xml files need to be copied into correct sub-folders in the folder for the mod, then those xml files can be edited.  Mods must be enabled in the game options.

There is a ModdingReadme.txt in the Mod directory that explains it more, but it refers to folder locations for the original GalCiv3, so adjust that to match what is correct for GalCiv4.

I wonder if you are looking at Documents/My Games/GalCiv4.  That is your personal directory and has nothing to do with the game files.

The game files are located where Epic installed them.  I'm not sure where that is for a standard install because mine are installed in a non-standard location.

Perhaps they are in C:\Program Files (x86)\Epic Games.  If not, you may have to do a search of your computer for GalCiv4.exe.

I'm glad you found them.

When you're making a mod, you don't need any folder that doesn't have anything in it.  I find it easier without all the empty folders cluttering up the folder for the mod.

I'm curious tried changing so settings in planetdefs.xml but it hasn't done a thing.  Kind of surprised there isn't an actual modding sub category in the forum.