It is difficult, of course, to know what Shakespeare would think of modern genetics, or even upbringing (though he does touch on analogous themes of birthright, nobility, and familial obligation in some of his other works). And scholars disagree about what, exactly, Shakespeare is trying to say about the relationship between free will and destiny in Macbeth. Since the question references Act IV, the most relevant scene (Act IV, Scene 1) seems to be Macbeth's second meeting with the witches. In this scene, the title character receives a series of visions, conjured by the witches, that make predictions for his future. The first warns him to beware the Thane of Fife, the second tells him to "scorn the power of Man" because nobody "of woman born" can harm him. The final apparition warns him that he cannot be conquered until Birnam Wood (a nearby forest) marches on Dunsinane. Obviously, these apparitions fill Macbeth with confidence about his future. But a final vision, that of Banquo followed by a series of kings, hearkens back to the original prophecy in Act I, which predicted that the descendents of Banquo would be kings. Macbeth acts on this advice, announcing in at the end of the scene that he will murder the family of Macduff, who has fled to England. It is perhaps in this scene that the interaction between fate, the supernatural, and free will is clearest. Macbeth is encouraged by at least some of the prophecies (though it could be argued that he hears the ones he wants to hear) and acts on them. On the other hand, it is up to Macbeth to act. The decision he makes to destroy Macduff's kin creates in the Thane of Fife an implacable enemy, fulfilling the prophecy. The witches did not force Macbeth to plot against Macduff. But it is difficult to say he had absolute free will in the matter either. So the question is here, as elsewhere in the play, left unresolved, though it is clear that supernatural forces in the form of the witches are exerting a malevolent force in Macbeth's life, exacerbating the effects of his own murderous ambition.