Moth comes to mind .
Mint
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
The letters ietremny do not all form one word and you cannot spell the name of an insect using all of the letters. You can spell mite using some of the letters.
A customary unit of length, which has 4 letters and starts with m, is mile.
metroid
magenta mustard
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