CitizenIV is the former concept of a GTA:O clone without much extensibility; it got replaced with the CitizenMP series of modifications which are more like a classic MP modification (servers, custom scripts, ...) except with the good parts of the old CitizenIV concept.
CitizenFX =/= CitizenIV, right? What is it, a multiplayer addon too?
CitizenFX is the name of the framework the CitizenMP series (which will probably see IV, MP3 and V versions) will run on, primarily to showcase that it doesn't only enable actual multiplayer-oriented interaction, but also 1-to-2-player SP-like experiences (with various additional engine features) or even every [SP base modification] player actually being in a multiplayer session (if they are able to connect) but in some sort of 'passive mode'...
Interesting, but how can we play GTA IV multiplayer without that s****y Games For Windows Live? Since rockstar doesn't give a single f**k about PC gaming community, they won't remove this DRM from the game, even after M$ shutting down their """""service""""" and GTA IV multiplayer will be unplayable :S