What does a small white pill with m 4 on it treat?

It seems like you are referring to a dilaudid 4 mg. it is a opiate/narcotic for moderate to severe pain. It is very similar to herion. EDIT: Small white ROUND M on one side 4 on other: Dilaudid 4mg as written above. Small white TRIANGULAR M on one side 4 on other: Fluphenazine hydrochloride 1mg. Used for psychotic disorders. Small white ROUND M (surrounded by a square) on one side 4 on other: Tylenol #4 (Acetaminophen with Codeine 300mg/60mg). Narcotic pain reliever used for moderate to severe pain. It *MAY* be Mesnex 400mg, which is a cancer treatment medication, however I have not been provided with an image for this one. So I can't say with confidence that it could be what you're looking at.