What are the 4 dynamic Italian markings?

There are actually five dynamic Italian markings, not four. They are:

1. Piano (p): Soft

2. Mezzo piano (mp): Moderately soft

3. Mezzo forte (mf): Moderately loud

4. Forte (f): Loud

5. Fortissimo (ff): Very loud

These markings are used to indicate the relative volume of music.