Answer:
The 4 Chords can be the 3 major chords in a key, aka the I IV and V chords and the relative minor aka the vi chord. For the key of C this would be C F G Am. If you know the key, the four chords are just the first, fourth, fifth and sixth. They could also be the 3 minor chords and the relative major aka the first fourth fifth and sixth of the minor key. For example, in the key of A minor this would be Am, Dm, Em and F#major.