You need to do it with a mod.
Go to Documents/My Games/GalCiv4 and open the Mods folder. Create a folder in the Mods folder and name it whatever you want. Open that folder and create a folder named Gameplay. Open the Gameplay folder and copy the GlobalDefs.xml file into it. Do all your editing in that file.
When you start GalCiv4, you'll have to go into Options and enable mods in the Gameplay tab. Exit GalCiv4, restart GalCiv4 and your mod should be active.
