Cloten has arrived in Wales, dressed in Posthumus's clothes and ready to do his dirty work:
Posthumus, thy head (which now is growing upon thy shoulders) shall within this hour be off, thy mistress enforced, thy garments cut to pieces before thy face: and all this done, spurn her home to her father, who may (haply) be a little angry for my so rough usage: but my mother, having power of his testiness, shall turn all into my commendations.The bit about the head is a nicely obvious bit of dramatic irony.
Over at Belarius's cave, Imogen, or rather Fidele, isn't feeling well, so Belarius, Guiderius, and Arviragus tell her to stay there while they go off hunting. The boys are getting strangely fond of their new adopted brother. Arviragus tells Belarius, "I know not why / I love this youth, and I have heard you say, / Love's reason's without reason." The feeling is reciprocated to some extent by Imogen, who is taken with their gentility: "Gods, what lies I have heard! / Our courtiers say all's savage but at court; / Experience, O, thou disprov'st report!" But before she goes into the cave, she decides to take the Queen's drug that Pisanio had given her.
Belarius and the boys are about to set off on their hunting when Cloten enters. He has gotten lost. Belarius recognizes him immediately, even though it has been twenty years since he saw Cloten at court. He immediately suspects that Cloten is with a party searching for them. Guiderius tells Belarius and Arviragus to go and scout out what others may be nearby, while he deals with Cloten.
Cloten challenges Guiderius as "a robber, / A law-breaker, a villain," but Guiderius throws the challenge back at him: "Say what thou art; / Why I should yield to thee." Cloten replies, "Know'st me not by my clothes?" which presumably means that Posthumus's clothes, which Cloten is wearing, identify him as a man of the court. But his dress means nothing to Guiderius, who replies, "Thou art some fool, / I am loath to beat thee." And when Cloten asserts, "I am son to th' queen," Guiderius doesn't care: "I am sorry for't: not seeming / So worthy as thy birth." Cloten threatens to post Guiderius's head "on the gates of Lud's town," and they exit fighting.
Belarius and Arviragus return, having found no sign of others supporting Cloten, and it isn't long before Guiderius rejoins them, carrying Cloten's head. Belarius is appalled: "We are all undone." But Guiderius says he'll take the head and throw it in the creek "and let it to the sea, / And tell the fishes he's the queen's son, Cloten." Belarius praises Guiderius's valor, but wishes he hadn't done it.
Still, what's done is done, so Belarius sends Arviragus back to the cave to check on Fidele while Guiderius takes care of Cloten's head. While they're gone, Belarius praises
these two princely boys: they are as gentleNature, it seems, has this time triumphed over nurture.
As zephyrs blowing below the violet,
Not wagging his sweet head; and yet, as rough,
(Their royal blood enchaf'd) as the rud'st wind
That by the top doth take the mountain pine
And make him stoop to th' vale. 'Tis wonder
That an invisible instinct should frame them
To royalty unlearn'd, honor untaught,
Civility not seen from other, valour
That wildly grows in them, but yields a crop
As if it had been sow'd.
Guiderius comes back from sending "Cloten's clotpoll down the stream," but suddenly there is the sound of what the interpolated stage direction calls "Solemn music." Belarius ascribes it to his "ingenious instrument," which is probably a wry Shakespearean pseudo-explanation of the use of offstage instrumentals. In any case, the instrument hasn't been heard since the death of Euriphile, whom the boys think of as their mother.
The reason for the solemn music comes when Arviragus enters with the "dead" Imogen/Fidele in his arms. Arviragus vows to "sweeten" Fidele's "sad grave" with flowers. Belarius remembers that they also have the headless body of Cloten as well, and should do it some honor: "Our foe was princely, / And though you took his life, as being our foe, / Yet bury him, as a prince." Guiderius agrees that "Thersites' body is as good as Ajax', / When neither are alive," so while Belarius goes to fetch Cloten's body, he and Arviragus sing the most familiar verses from Cymbeline, the dirge that begins "Fear no more the heat o' th' sun."
They lay Imogen and the headless Cloten side by side and scatter flowers over them, then depart. As soon as they have gone, Imogen awakes, somewhat deliriously thinking that she is asking her way to Milford-Haven, but soon realizing that she is lying on the ground, covered with flowers, next to a headless corpse. She begins to remember that she has been "a cave-keeper, / And cook to honest creatures." And then she recognizes Posthumus's clothes on the body, and persuades herself that its arms and legs are those of her husband. She blames Pisanio, accusing him of conspiring with Cloten, of forging Posthumus's letters, and of drugging her. Smearing Cloten's blood on her face, "That we the horrider may seem to those / Which chance to find us," she falls on the body in a paroxysm of grief.
Now Caius Lucius, accompanied by his captains and a soothsayer, enters to plan the attack on Britain at Milford-Haven. A captain assures him that soldiers commanded by "bold Iachimo" will soon arrive, and the soothsayer claims that success is at hand:
I saw Jove's bird, the Roman eagle, wing'dBut Lucius interrupts this questionable interpretation of the soothsayer's dream because he has spotted Cloten's body with Imogen lying atop it. She rouses herself from her swoon to tell him that the body is that of her "master, / A very valiant Briton, and a good, / That here by mountaineers lies slain." She decides not to tell Lucius the truth, and claims that her master was Richard du Champ, though she continues to call herself Fidele.
From the spongy south to this part of the west,
There vanish'd in the sunbeams, which portends
(Unless my sins abuse my divination)
Success to th' Roman host.
Lucius asks if, since her master is dead, she will join his service, and she agrees. First, however, she asks that they bury him, and they take Cloten's body away for interment.
Back at Cymbeline's palace, we learn that the Queen is so upset at Cloten's disappearance that she has a "fever.... A madness, of which her life's in danger." Cymbeline reflects on this sorrow, added to Imogen's disappearance and the threat of war with Rome. He threatens to torture Pisanio to find out if he is really ignorant of Imogen's whereabouts. Pisanio assures him he is, and the First Lord testifies to Pisanio's loyalty. Meanwhile, the First Lord says, "The Roman legions, all from Gallia drawn, / Are landed on your coast." Regretting that loss of "the counsel of my son and queen," Cymbeline goes off to make preparations, and Pisanio reflects that he hasn't heard from either Posthumus or Imogen, and prepares himself to fight against the Romans.
Back at the cave, Belarius is trying to persuade Guiderius and Arviragus to go higher into the mountains to avoid the conflict between the Britons and Romans. He tells them that there are "many in the army" who would recognize him. But Guiderius wants to join the fight, assuring him that so much time has passed that he won't be recognized, and in any case, "I and my brother are not known." And Arviragus wants to fight, feeling that his manhood has never been tested in battle: "I never / Did see man die, scarce ever look'd on blood, / But that of coward hares, hot goats, and venison!" Both of them beg Belarius to give them leave to fight in the war, and he reluctantly agrees.
Act IV of this 1982 BBC-TV production begins at about 1:40:18.
Richard Johnson as Cymbeline; Helen Mirren as Imogen; Michael Pennington as Posthumus; Claire Bloom as the Queen; Paul Jesson as Cloten; Robert Lindsay as Iachimo; John Kane as Pisanio; Hugh Thomas as Cornelius; Geoffrey Lumsden as Philario; Patsy Smart as Helen; Alan Hendrick as the Frenchman; Graham Cowden as Caius Lucius; Michael Gough as Belarius; Geoffrey Burridge as Guiderius; David Creedon as Arviragus; Marius Goring as Sicilius Leonatus: Michael Hordern as Jupiter