There are three notes to learn on the fourth string, the open D note, the E note on the second fret and the F note on the third fret.
The open fourth string is a D note. This D note is written in the space below the staff.
The 4th, 5th and 6th strings are picked with the thumb ( p ). In almost all cases a free stroke is used. After striking each D note in the following exercise the thumb should move over the third string and return to its original position ready to strike the D note again. Your thumb should feel as if it is moving in small circles.
The note on the second fret of the fourth string is an E note. Use the second finger of your left hand to fret this note. This E note is written on the first line of the staff.
The note on the third fret of the fourth string is an F note. Use the third finger of your left hand to fret this note. This F note is written in the first space of the staff.