Hyrule Field - Faron Province Piece of Heart
The fourth piece of heart you come across is located up in a tree in of Hyrule Field. The tree is just to the right of the bridge nearby two raised land masses - also very close to the first Golden Bug you come across in the game. Additionally there will be two goblins that attack you when you get close to this area.
Once you spot the piece of heart you'll need to use your Gale Boomerang to get it down from the branches. Piece of cake!