Avatar 2 and 3 are likely connected in some way, 3 being the climax. But 4 and 5 astound me. They have to be concepts for a streaming series or event show.
People will tire of the thing after two more films... although I believe most are tired now. Perhaps #2 can gain some curiosity and rejuvenate moderate interest, but #3 will suck up any that is generated. Then he's got #4... then #5.
If he tries to connect ALL the films, then he's going to drag the story out so much that the first two kill any chance of getting to 5 (like Alita)... if the first two are connected into a two-parter, like IW/EG, then he has to reset everything for another two movies after that... which as we can see with the MCU is no simple feat.
I have no doubt he'll do a damn good job with whatever he's got, but he's not a great writer and the world is not that interesting (so far) to sustain 4 more movies. The first movie couldn't sustain any interest beyond its initial 3D gimmick -- there's no longevity in rewatching that world for almost anyone. And that's the difference. That's what keeps bringing people back to Star Wars. The need to revisit that universe.
Perhaps Cameron will open up his world into a fuller universe. Otherwise, its like starting off Star Wars in 1977 on Endor with the Ewoks. I doubt that would have jump-started Star Wars like we know it today.
We shall all see...