What happens
In the forest after battle, Falstaff captures the rebel knight Colevile, who yields upon recognizing Falstaff's name. Prince John enters and dismisses the troops, then mocks Falstaff for arriving late. Falstaff boasts of his capture and claims credit for the victory. Lancaster praises the chief justice and prepares to ride to court with news of the king's illness. Falstaff asks permission to visit Gloucestershire before following.
Why it matters
This scene reveals the hollow nature of Falstaff's martial prowess. Colevile surrenders not because Falstaff is a skilled warrior, but because his reputation precedes him—the knight 'yields' the moment he realizes who he faces. Falstaff immediately spins this moment of pure luck into a heroic deed, claiming he has 'in my pure and immaculate valour, taken' Colevile. Lancaster sees through this immediately, noting that Colevile's surrender 'was more of his courtesy than your deserving.' The scene exposes Falstaff's gift for self-aggrandizement: he transforms a trivial event into personal glory, demonstrating the gap between his claims and reality that will soon destroy him.
More significantly, this scene marks the beginning of the political reversal that will define Act 5. Lancaster's entrance brings news of the king's dangerous illness, shifting focus from battlefield to court. While Falstaff still basks in imagined glory, Lancaster is already thinking about succession and governance. Falstaff's request to visit Gloucestershire—where he plans to exploit Shallow and his lands—shows him preparing to consolidate power through local influence, confident that his newfound glory will translate into authority. This confidence is entirely misplaced, as the scene subtly foreshadows: Lancaster's cold dismissal of Falstaff's boasting, and his stern focus on duty, preview the new king's very different values.