Lowercase "a" in organic chemistry nomenclature?
'4a' and '8a' are locants which refer to a specific atom. They are fundamentally no different from the numbers '1', '2', '3', '4'... which are also locants referring to specific atoms.

The locants in geosmin come from the locants in naphthalene, which are assigned based on the following excerpt from Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book):


  P-25.3.3.1.1 The numbering of peripheral items in the preferred orientation starts from the uppermost ring. If there is more than one uppermost ring, the ring furthest to the right is chosen. Numbering starts from the nonfused atom most counterclockwise in the ring selected and proceeds in a clockwise direction around the system, including fusion heteroatoms but not fusion carbon atoms. Each fusion carbon atom is given the same number as the immediately preceding nonfusion skeletal atom, modified by a Roman letter 'a', 'b', 'c', 'd', etc.