The model number (GN-4WS12G) is on the caseback, but the is also engraved there. Look for a 4-digit number followed by a star or dash (e.g., **E168** , **E690** , **H500** ).
Look at the back of your watch for a string of characters like XXXX-YYYYYYY . citizen eco drive gn4ws12g manual link
Since these watches are powered by light, they do not require battery replacements. If your watch has stopped or is losing time, Citizen recommends exposing it to for several hours. The model number (GN-4WS12G) is on the caseback,