Prospero in Act 4 is very fatherly like. He is too over protective. Nevertheless he issues them a warning. If Ferdinand takes Miranda's virginity before their ceremony, their union would be cursed. Ferdinand, of course, swears he won't do anything not until the day of their ceremony. Prospero calls upon Ariel to perform one of his last acts of magic. Prospero's decision to let Ferdinand and Miranda marry was made even before Ferdinand came into the island. Miranda's virginity is very important. Why? Back in their time, a woman's virginity was so golden that it was important it gained a great deal of power throughout her reign. So Prospero tried his best to keep Miranda well informed of her importance. Prospero suggest that while he keeps Miranda's virginity he will be able to get back to Italy and gain back his power. If Miranda's virginity is thrown away, then Prospero's greatest hope of regaining his position is gone too so he is very careful with Miranda. Don't you see? He keeps her hidden. She has no idea there are more people out there in the world than Ferdinand and her father. Prospero keeps Ferdinand warned as to the consequences to his actions.
Prospero uses a metaphor to compare his daughter to something "rich" to exchange. Prospero's metaphors could be meant to distract Ferdinand from what Prospero and Miranda are getting in the bargain.
Prospero has everything under control! He wanted revenge right? Well, he really doesn't give that.
I think Prospero's actions in the final act reveal his true character. Prospero actually forgives. He makes up his mind about revenge and makes a speech that signifies his formal rejection of magic. Prospero charges Alonso for throwing Prospero and his daughter out of Italy, and Sebastian and Antonio for being part of this crime. Prospero announces Ariel's freedom and Ariel sings a song out of joy. Prospero's judgement on his enemies generally have been justified. Gonzalo was always a honorouable man. Caliban, Stephano, and Trinculo are too drunk to even get punished. But Prospero's character is changed in the last act so much he forgives those that gave him wrong. Caliban apoligizes Prospero for taking Stephano's foolishness and Prospero acknowledges Caliban and takes him as his own.
Prospero's first words suggest a metaphor, the words "gather to a head" suggest things coming to a cimax.
There are also a ton of allusions Prospero puts in his speech such as "green sour ringlets". These ringlets are small circles of sour grass caused by the roots of toadstools, made by fairies dancing.
When Prospero at last confronts Alonso and his brothers, he uses another metaphor to demonstrate the gradual process of Prospero's spell fading from them. Prospero gives up his magic something ironic in this act. A major theme running though the entire work is forgiveness versus vengeance.
