A 'myth'.
"Myth" is the closest answer, although strictly speaking, it is not a synonym of fable, as it does not mean the same thing.
A synonym for fable (4 letters, beginning with M) is myth.
myth
myth
myth
Myth is a synonym of fable. Both refer to traditional stories that are often used to explain natural phenomena or convey moral lessons.
probably "myth"
fable
Mythical Man.
Doctrine, philosophy
fable, narrative, saga, fiction, tale
announce, speak out, and affirm are some good ones.