Grand Theft Auto Episodes Questions?

I haven't played GTA4 in a while,but now something inside of me wants to play it again. With the new content coming out for it I was wondering if u can play the stories simutaneously or do have to beat Niko's story, then the other stories will unlock accordingly?

You DO need the GTA 4 disk if you have downloaded through PSN as a DLC. If you have the physical copy of Episodes, you do NOT need the GTA 4 disk.