Yes to all of the above but I've never heard it called the 4th density. I think we are talking about the same thing and I know it as the astral plane. The astral plane isn't a final destination "when we die" though, I think of it as the lobby. There are many levels to the lobby and there are "rough areas" and exquisitely luxurious areas. The mind-set and 'vibration/frequency' of the person departed/vising determines which area they congregate towards. They are free to increase their vibration/frequency to travel "up" if they desire.