The fourth finger can be used to play notes on all four strings. In the following example, it is used to play an E note on the A string instead of using the open E string.
This is indicated by the fingering number (4) written above the E note. The fingering for this whole piece is identical to the previous example except that it is played on the A string instead of the E String. The melody is the same, but it sounds lower because it is on a lower string.