After you complete Dungeon 4: Snake, you should have enough heart containers to get the Magical Sword, which is the strongest sword in the game. This page explains how.
Have Twelve Heart Containers
If you have 12 heart containers, you can get the magical sword. If you have completed four dungeons, you should have at least seven heart containers. You can get five more if you have the Raft, Stepladder, Blue Candle, and Bombs. Read Heart Containers for more details. The Blue Candle can be purchased from some shops, and you can fight enemies like Blue Octoroks and Blue Moblins for a chance to get bombs.
Get the Magical Sword
Once you have twelve heart containers, you need to go to the graveyard in
west Hyrule. To get there, go to southwest Hyrule until you reach a red
forest where the path is shaped like a cross. Once you are there, kill the
Blue Moblins, then go North, West, South, West. You will end up in a new
area. Go north twice to reach the graveyard. Then go to the northeast corner
of the graveyard and push the grave that is marked with an X below:
O O O O O O X O O O O O
Stairs will appear. Go down and get the Magical Sword. If nothing happens when you walk over the sword, then you don't have enough heart containers! Read the previous section for information about getting enough heart containers.
Go to the Next Dungeon
If you have been doing the dungeons in order, you should now go to Dungeon 5: Lizard.