Simply try out your console as a MIDI controller outside of VDJ to verify if indeed the button is at fault or not:
I have checked the midi controller and all the buttons are OK.
Was this with the latest Virtual DJ v7.3 ?
Are you using any skins or mapper plugins that could potentially add or modify the behavior of the browse buttons ?
Also keep in mind that in the default MIDI mapping for the 4-Mx, when you hold the FOLDER or FILES buttons, the jogwheels can be used to fast-scroll through files, folders or playlist, so this could lead to confusion if you were not aware of it.
You may want to ask Virtual DJ support then. They might be able to look into it with you.
Thanks Dj Phatso.