The potion symbolizes the turmoil in Macebth’s soul, and the ingredients represent the elements of the witches’ plan pushing him to his destruction.
Throughout the play Macbeth, the witches are symbolic of Macbeth’s deep and dark desires. In a way, they symbolize the darker side of humanity as a whole. Act 4, scene 1 is important because the witches are stepping up their game. They need this potion, their stew, to be extra effective so they can continue to manipulate Macbeth. Symbolically, the growing number of ingredients each represent one more element of Macbeth’s downfall by his greed and distraction. Hecate is pleased with the “stew” and tells the witches:
O, well done! I commend your pains,
And everyone shall share i’ the gains.(enotes etext p. 58)
By everyone, she means the audience as well as the witches. The audience is in for the treat of Macbeth’s total destruction and downfall. The potion symbolizes the inner turmoil of Macbeth’s soul, and it is very steamy by this time!
For the etext pdf, click here: http://www.enotes.com/macbeth-text