"Four eyes" is a negative slang description for a person wearing glasses.

Trying to visualise the mentioned "four eyes" leads to the assumption of the origin. The "four eyes" mentioned could come from the person's two eyes plus the two lenses fitted into the glasses, or it may come from the optical illusion created when the glasses are down the nose a bit at just the right angle for refraction in the glasses to show another pair of eyes under the person's normal ones. However the term "four eyes" is so synonymous with "a person wearing glasses" that one would not often stop to consider where the supposed four eyes as mentioned comes from.

The term carries negative inferences and is often used by bullies, however it is such a widely-known degrogative term that the speaker usually just uses it, trusting that it is a bad thing to call someone, and is highly unlikely to consider what they are specfically inferring when using the term. "Four eyes" is generally associated with being a geek or nerd, who are often stereotyped as wearing glasses. A geek or a nerd 20-30 years ago used to be a person that was also mocked, however as our culture is changing to accept geek and nerd pursuits as more mainstream, this has slowly changed so that being a geek or a nerd is no longer seen as such as bad thing. However the term "four eyes" is firmly connected subconciously to the older mocked version of geeks and nerds. Similarly, a person called "four eyes" is demoted to being thought of as weak. They are dismissed as unlikely to join in with rough sports or activity and not part of the jock/popular group, and therefore a target.

"Four eyes" is still currently used derogatively. As with many bullying terms, the words hurt even when the inferences are not connected with truth or reality. Of course anyone can need glasses, not just those academically or special interest inclined. Of course people needing glasses are just as likely to pursue sport as a person without glasses.