Mint
magenta mustard
Moth comes to mind .
A synonym for fable (4 letters, beginning with M) is myth.
Are you sure it ends in 'o'? A muon is a four letter subatomic particle.
meshmohairmilliskin
A customary unit of length, which has 4 letters and starts with m, is mile.
metroid
Motorola
The distinct letters in the word "MISSISSIPPI" are M, I, S, and P. There are four unique letters in total.
Those six letters are "amount."
molecular
M t gg