In Macbeth Act IV scene i, describe the role of the supernatural elements of the scene.

In addition to the excellent answer above, the
supernatural in act 4.1 of Macbeth serves functions such as
contributing to the play's imagery and adding
unity.

Blood is often present in
Macbeth, from the description of the battle by the bloody Captain
in Act 1.2 to the bloody dagger Macbeth envisions in Act 2.1 to both Macbeth and Lady
Macbeth obsessing over blood on their hands, etc.  In Act 4.1, blood
imagery is contributed to in the form of the Bloody Child. This Second
Apparition also adds to the play's imagery of children: 
most notably, the slaughter of Macduff's son in Act 4.2, in which imagery of blood and
imagery of children come together, just as they do in Act
4.1.

Repetition adds
unity to any work, of course, and this play is no
exception.  The theme of the supernatural is repeated in
Act 4.1, after having been first introduced in Act 1.1, as is the capacity of the
witches for making predictions.  The theme of
equivocation is repeated here, as well, which will become
evident later in the play.  The idea of Banquo's heirs
reigning is repeated and made concrete by imagery, as well.  And, of
course, the imagery of blood and children adds
unity.

These are some of the roles, or functions, of the
supernatural in this scene.