In Lost Ark, there are different kinds of collectibles that you can search for to receive rewards. One of these collectibles is called Giant's Hearts. They look like gems and are described as the heart of a giant who fell during the Hollow War. These collectibles can be acquired through various means, as outlined in the Collectibles list. You can also use them to unlock different rewards by visiting Wisdom Isle and speaking with Minerva.
The Fourth Giant's Heart is one of the easiest you can get. In order to acquire it, you must go to Lake Eternity in Shushire.
This Giant's Heart is a random RNG drop from field boss Tarsila in Lake Eternity. This boss takes a long time to spawn and does so randomly according to Procyon's Compass. So, you need to make sure that you check it beforehand. Tarsila is a large scorpion boss and has a total of 28,789,096 hit points.
When you do acquire the Giant's Heart, you don't need to select it in your inventory or do anything to bind it to your character to receive it.
Want to see more about classes in Lost Ark? Check out How to lock items in Lost Ark on Pro Game Guides.