Any time a real whole number is specified for <NumBuildingNeeded> under the <Building_PrereqBuildingClasses> table, the game will double the number entered in the XML to determine how many of the <BuildingClassType> a player will actually have to construct before the building stated in <BuildingType> can be built. The count the game keeps of the required number of buildings does not care one way or the other about the capital. It is merely looking (in your case) for 2 * 2 = 4, or 2 * 3 = 6.
see link.
Thanks, I've been wanting to mod all National Wonders to a flat 4 needed per empire for years. Weird that you have to set it to one half of the desired value.