In Shakespeare's play The Tragedy of Macbeth, the four men who lead the army against Macbeth in Act V Scene VI are Macduff, Old Siward, Malcolm, and Young Siward.
Old Siward is a general in the English forces. In Act V, Scene II, readers learn from Menteith that Old Siward is Malcolm's uncle.
"The English power is near, led on by Malcolm,
His uncle Siward and the good Macduff."
In Act IV, Scene III, Malcolm tells Macduff about Old Siward's march with his impressive army:
"Whither indeed, before thy here-approach,
Old Siward, with ten thousand warlike men,
Already at a point, was setting forth."
Malcolm is the son of King Duncan
and the rightful heir to the throne. He had fled Scotland under suspicion of murder when his father was killed.
Macduff is the Thane of Fife and is the one who eventually murders Macbeth. Macduff tests Malcolm to see whether he can tolerate him as a replacement to Macbeth before joining forces with him.
Young Siward is the son of Old Siward, the general, and is part of the army that comes against Macbeth. The way that they come against Macbeth in Act V, Scene VI, fulfills the prophecy from the third apparition that stated that Macbeth would not be defeated until Birnam Wood moved to Dunsinane. Macbeth believes this is impossible, so he gets overconfident and complacent, allowing the forces led by Malcolm, MacDuff, and Young and Old Siward to overtake him. Here is the prophecy from the third apparition from Act IV, Scene I:
"Be lion-mettled, proud, and take no care
Who chafes, who frets, or where conspirers are.
Macbeth shall never vanquished be until
Great Birnam Wood to high Dunsinane Hill
Shall come against him."