So this movie got pushed back by 3 years to 2029 with Avatar 5 also getting pushed back 3 years to 2031.
If I had to guess, aside from the Writers Strike making things difficult, they probably plan on filming Avatar 4 & 5 back to back similar to how they did 2 & 3. If i'm correct that they are filming back to back, then that would make sense why there's a four year gap between Avatar 3 and Avatar 4.