Four-leaf clover is an Elona+ exclusive food item. Eating it trains luck potential and gives some experience toward base luck increase.
They can be found by mining the forest walls, in forest themed dungeons. Just like Acorns, and Golden Acorns. They are a rare find (1/200).
The item sprite ID for a four-leaf clover is 64 (Row 1, Column 31) using the item sprite zero-based position system.