On any Harmonica, first position and fourth position are closely related. On the C harmonica, first position is used for playing in the key of C Major, while fourth position is used for the key of A minor. These are called Relative keys. Both keys contain the same notes, but their scales start in different places, as shown below.
Here are the scales of C major and A natural minor. Notice that it is basically the same scale to play, except for the starting and ending notes.
Many songs move between a major key and it’s relative minor, or vice versa as demonstrated here. The backing to this example has been recorded as a Jam-along track at the end of CD2 (page 178) so you can practice alternating between relative major (1 st position) and minor (4th position) keys.