Civ 4 is the last in the series that does not have workshop compatibility on Steam. It's too old and primitive for the workshop I guess, meaning people who want to mod it have to come to websites like CFC if they want to download. Civ 5 on the other hand has it's mods directly available through the workshop, so despite being more popular it's fans don't ever have to go out into the wider internet to mod.
Forums as a medium are a dying breed well beyond the trials and tribulations of Civ.