Piano score for 4 voices?

No.
You need a separate staff for each voice. Add new piano staves and use Tools/Explode.
But 'Explode' has its pitfalls: it distributes the voice to the staves directly below it in the score, no matter what the staff is there, so it would write voice 2 in the bass part. Therefore, you should add a separate piano instrument for each of the additional soprano voices and the bass voices. Yes, it is a bit cumbersome.
If you do this in a copy, your original score will remain unchanged and you can generate the mp3 from the copy.

To make each voice into an mp3...
You can untick 'Play' in the Properties panel for the voices you do not wish to hear in the mp3.

So, for example, to isolate Soprano (on the top piano staff):
Select all notes in the bass clef staff and silence all of them via unticking 'Play' in the Properties panel. This will leave only Soprano and Alto sounding on the treble clef staff.
Use the selection filter to select only voice 2 (Alto) and untick 'Play'. This will leave only Soprano sounding - which you can export as mp3.

Do similarly to isolate and export each of the other voices as mp3.

Note: An alternative to the selection filter is to right click a note then Select > More.. and choose 'Same staff' and 'Same voice'.