The occurrence of fourth molars in mandible is very rare. A 32 year-old male patient applied to our clinic to seek treatment for a pericoronory abscess distal to the left lower third molar. Radiographic examination revealed an impacted fourth molar located in the coronoid process and a well-defined radiolucency extended downward and forward to retromolar region. Under general anesthesia both third and fourth molars, and the radiolucent cystic lesion between them were surgically removed.