Dungeon 4

 Go to the room on the left, and you'll find that it is pitch dark. The way to light these rooms is to use your Candle. Now collect your Compass and leave this room.
Now go up. Don't forget to defeat all the enemies here and grab the Key.
In this room, you'll win a Bomb as a prize for beating all the baddies. This is usually a hint the game gives, indicating a false wall is nearby. Bomb the wall on the right.
A shiny new Key!
Go back to the left and up a screen. Now go to the right and pick up the Map.
Go left again, then up. When you reach the Moldorm, you must make a choice: to fight, or not to fight? The Moldorm is pretty easy, just slash at its tail and kill it segment by segment. After beating two Moldorms, you'll be rewarded with a Blue Rupee. ...Eh, maybe it's better to just avoid fighting them.
These skull blob things are Bubbles. Touching them will render you inable to use your sword for a few seconds. They also can't be defeated, so don't even try.
When you reach this room, head to the north (not like you have any choice without any keys.
Like-Likes! Be careful around the Like-Likes, especially if you have the Magic Shield, since they will permanently eat it. The room to the right has the Old Man, and he's boring, so let's go left instead.
A staircase! But let's bomb this wall first instead...
This guy will give you the ability to carry 4 more bombs, all for the low, low price of 50 Rupees. This is a bargain, so don't pass it up.
Go back and defeat all the Darknuts to win a Key and access to the staircase.
The Ocarina! This will certainly come in handy...
With the new Key you just collected, return to the room with the Vires and unlock the door. Take this staircase.
When you come out, you'll be in a room with a new enemy, the Pols Voice. These guys are tough, but playing your Ocarina will instantly (and cheaply) defeat them.
The boss is the Digdogger. The first thing you must do is play the Ocarina. This will split him into 3 mini-Digdoggers. Try to attack from a distance. If you have the Arrows, that makes this battle a bit easier.
Another Triforce Piece bites the dust.