overlaying tracks

I am new to using Audacity. How do I overlay 4 tracks so that they play simultaneously. I have each track separately recorded and saved. Also, how do I import an mp3 file to play simultaneously over the verbal track? Thanks

You can drag-n-drop multiple tracks into Audacity …
Drag-n-Drop files into Audacity.gif
The tracks will play simultaneously.
With multiple-tracks you’ll probably have to reduce the gain-slider on each of the tracks,
as otherwise their combined volume will be too much and cause clipping distortion.

With multiple tracks playing at the same time you’ll probably be in need of Audacity’s envelope tool to vary the volume …