Only if you completely redefine some aspect of the equation. You'd have to define "5" to actually mean "4" or change the meaning of "+" or "=" in some way that changes the operation. 2+2=4 isn't just an abstract statement, it's based on the way the physical world works. If you have 2 apples, and then I give you 2 more, you don't suddenly have 5 apples because we all decided 2+2=5.
Orwell's meaning in 1984 wasn't about belief changing the world, it was about the power of brainwashing and how fascism demands obedience.
You have 2 apples. I give you 2 more. How many apples do you have? Unless you redefine what the numbers or the operators mean, then you now have 4 apples. That's a truth that is evident in the world and can be verified. That's what a fact is.
He didn't suggest we could never determine the age of the moon. He said that science refines it's methods and gathers new information, and so we may change our estimate of its age based on new evidence.