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Frist of all it might be m4b rather mb4.
iTunes has a built-in converter. Frist open iTunes > Preferences. Look for the Import Settings button. After clicking, it opens a list of the various formats that iTunes can use. Select MP3 Encoder & set the quality to the level as desire. Next click OK. Now after that, on the main iTunes page click on the Music button in the upper left corner & select Audiobook/ m4b. Open Library & go to your current collection of titles. Select this one to convert then highlight the track itself. Now, locate the Menu bar & select File > Convert > Create MP3.
This procedure may vary on iTunes version.