After getting the third Triforce piece in dungeon 3, you can head to Dungeon 4: Snake.
Optional: New Heart Container
With the Raft found in Dungeon 3, you can now get a heart container that was previously inaccessible. Go to the southeast part of the map, then go north (ignore the heart container that is on a platform; you can't access it yet). Eventually you will reach a small dock. Walk north onto that dock and keep going north. You will automatically get on the raft and go across the water to a new area. Go into the cave and take the piece of heart.
Location of Dungeon 4
Dungeon 4 is slightly south and west of the center of the map. From the place where you started the game, go west, north, west, and north to reach a small dock. Walk north onto that dock and keep going north. You will automatically get on the raft and go across the water. The dungeon is here. Go in.
The Quick Way
If you don't want to collect everything in this dungeon and just want to get the important items quickly, read this paragraph. Otherwise, skip to the next section. Go north twice from the entrance, get the key, then go west, north, and get the key. Go north again, then go through the locked east door, then kill the Vires to open the east door. Kill all the monsters in here (be careful of the Like-Likes, which can eat you and take your Magical Shield if you bought one), then push the left block and go down the stairs to get the Stepladder. Go back upstairs and go west twice, then walk across the water (the stepladder lets you do this) and go north through the locked door. Go east and then bomb the middle of the north wall, and go through the hole that you created. Then bomb the middle of the east wall and go through that hole. Kill the Vires in this room, then push the left block to open the east door. Go through it and attack Gleeok's heads until he is defeated. Go north to get the Triforce Piece.
Get a Key
Go west from the entrance and kill all the Keese to get a key.
Get the Compass
Go east from where you got the key, then go north, then go east through the locked door. If you have the Blue Candle, you can use it to light up this room. Get the Compass from the middle of the room. (If you don't have the Blue Candle, try going two squares south of the door, then go east until you can't anymore, then go south, then east until you can't anymore, then go north until you have the Compass.
Get a Second Key
Go west from the room where you got the compass, then go north. Get the key.
Get a Third Key
Go west and use the Blue Candle to light your way. Then go north and use the Blue Candle again. Get the key in this room.
Get the Stepladder
Go north and use the Blue Candle again. Your path is blocked by some water. Go east through the locked door.
Kill the Vires in this room to make the east door open. Go through.
Be careful of the Like-Likes in this room. They can eat your Magical Shield. If they do, you will have to buy another one from a shop somewhere. Anyway, kill all the enemies in this room and then push the left block. Stairs will appear. Go down them, then get the stepladder.
Get the Map
Go back upstairs, then go west. You can walk over the water using the Stepladder, so do that to go west again quickly. Use the Blue Candle if you need to, then go north through the locked door.
Go east from the room full of Vires.
Use the Blue Candle. Go around the edge of this room to get the map. Then bomb the middle of the north wall and go through the hole. You will find ten rupees.
Go to the Dungeon Boss
Now bomb the middle of the east wall and go through the hole. Kill all the Vires here and then push the left block to make the east door open. Go through and you will reach the dungeon boss.
Gleeok is a two-headed monster that spits fireballs, but the Magical Shield doesn't block these fireballs, unfortunately. Attack the heads with sword beams, or use your sword at close range if you have to. Arrows and bombs won't hurt it. Once a head is killed, it turns red and floats around, spitting more fireballs. Kill the other head as quickly as you can. Then get the Heart Container that Gleeok leaves behind.
Get the Triforce Piece
Go north from where you fought Gleeok and get the Triforce Piece.
Now that you have the step ladder, you should be able to get at least twelve heart containers total. Because of that, you can get the Magical Sword. You should do that before the next dungeon.