Civ 4 uses the .nif format for it's 3d models which is the same used by Bethesda games, so it should theoretically be possible to export the body mesh from some sexy NPC replacer mod for Skyrim and put it in Civ 4 for even sexier custom leader models, although the skeletons are probably different so the animations wouldn't work. And if the leader animations are pre-rendered it wouldn't be possible anyways.