The patient went on to have surgery which confirmed the pre-operative diagnosis of an epidermoid cyst. The 4th ventricle is actually not that uncommon a location for this entity, but far less common than the cerebellopontine angle. 

Histology

MICROSCOPIC DESCRIPTION: The sections show the lining and contents of an epidermal cyst. The lining is composed of squamous epithelium with a discernible granular layer. The contents comprise laminated keratin. No mesenchymal or appendageal components are identified. There is no evidence of tumor.

DIAGNOSIS: Epidermoid cyst