You need the search sphere. Have you rescued the tourists in the Calm Lands at Yojimbo's Fayth Cave? If you have, one of the tourists gives you the Besaid Key, which opens the locked chest in the temple. There is a search sphere inside the chest. Use the search Sphere to find the 4 numbers and you're set! The Besaid Key was 900,000 Gil in Chapter 1 but she sold it to a tourist, the tourist gives it to you free of charge when you save him in the cave.