Enter Horatio, Gentleman, then Sailors:
In this very short scene a gentleman brings "sea-faring men" (4.6.2) to Horatio. One of the sailors has letters from Hamlet. One of the letters is to Horatio, and he reads it aloud.
Hamlet writes that his ship was attacked by pirates. In the battle, Hamlet boarded the pirate ship, and at that moment the pirate ship got clear of Hamlet's ship. One might expect that the pirates would want to hold the prince for ransom, but Hamlet promised the pirates to do them a "good turn" at some future time, and so persuaded them to bring him back to Denmark.
Rosencrantz and Guildenstern are still headed for England. Horatio is to deliver Hamlet's other letters to the King, and then come meet him. Hamlet has astonishing news for Horatio.