If you already found the 4 #'s, sounds like you did, by the way I think mine were 5846 or close to that, Then you need to find the cave. Past the waterfall your location should say Besaid Crossroads on the menu screen - you should see a little red thingy, walk down past this and find red steps, This is the cave. You'll be asked to input your #'s and then you'll be able to enter the cave.
I had a hard time with this too. Other people can help besides just Gerbill <img src="/images/graemlins/tongue.gif" alt="" /> , Although you have to learn patience and you may have already found the cave by now.
I also recommend using this little area to level up, you'll need it. I came back here after I got my white mage dress sphere, (The only thing is I don't remember where I got it from) and leveled up one white mage.