The 4th, 5th and 6th strings are picked with the thumb (p). In almost all cases a free stroke is used. After striking a string the thumb should move over the third string and return to its original position ready to strike again. Your thumb should feel as if it is moving in small circles. Once again consider the two options for the position of the right hand thumb.
The classical position: The wrist is arched, thumb 45 degrees to the string and the thumbnail is used to pick the string.
The contemporary position: The wrist is kept close to the guitar face, the thumb almost parallel to the strings and the side of the thumb is used to strike the string.