Capri is very easy to do on your own. The ferry takes about twice as long as the hydrofoil (45 min. v. about 90). The 4:30 return would be fine if it leaves on time; but that's a big IF.
Just a suggestion: we took a ship tour that started with the hydrofoil to Capri; then another to Sorrento for sightseeing and a great lunch at a seaside reataurant. Then to Pompeii for a guided tour, and back to the ship. It wasn't cheap, but we felt it was well worth it--no worries about schedules, currency, ets. Just a thought.