What literary devices are used in Macbeth, Act 5, Scene 4?
In Act 5, scene 4, Malcolm describes the fact that Macbeth seems to want them to lay siege to his army at Dunsinane castle. He says, "none serve with him but constrainèd things / Whose hearts are absent too" (5.4.13-14). In other words, no one serves Macbeth any longer unless they have no choice, and, even then, those men do not serve him with their hearts. He means that no one feels loyalty or love for Macbeth at this point, but he uses "hearts" to stand in for those emotions: this is an example of metonymy, when an author uses something associated with the thing he means for the thing itself.
Further, Siward uses personification when he says, "Thoughts speculative their unsure hopes relate, / But certain issue strokes must arbitrate" (5.4.19-20). He is personifying "thoughts" as being able to have hopes and "strokes" as being able to arbitrate conflict. What he means is that it is easy for them to sit around and have high hopes for the coming battle (when they are not actually engaging in that battle yet, this is why the thoughts are "speculative"), but that sometimes the only way to resolve a conflict is through violence.
Finally, Shakespeare uses this short scene to increase the pace of the action. Short scenes like scenes 4-8 in this act, have the effect of seeming to speed up the action, and this increases tension for the audience as the plot builds toward the climax: the fight between Macbeth and Macduff. By controlling the pacing in this way, Shakespeare heightens tension for the audience.