James Cameron was bluffing. Avatars 4 and 5 were always the plan.

I firmly believe that the whole "Avatar: Fire and Ash will be thew last one if it doesn't earn enough" was always a marketing pitch. Cameron only said that to scare the fans into supporting the series more, make the non-fans feel like this installment in the franchise has a special importance because it can be the final one, and overall put butts in seats.

It's hard to say if it worked or not. There is no way to find out if the numbers were affected in any way or not, but I think that's what Cameron was counting on.

His claim that Fire and Ash is supposed to serve as a satisfying finale, in case there is no more movies... is just bullshit. Nothing was really resolved. Mysteries were not answered. Loose ends were not tied. This is a textbook mid-point of a series, not a finale, and it was never conceived as such. More movies were always a part of James's plan, and him saying otherwise was a lie.