"Four-eyes" is an English phrase that is normally used to identify people who wear corrective lenses in order to enjoy a higher quality of vision. Typically, the idiomatic expression is applied in a derogatory manner rather than as a means of identifying a positive attribute. "Four-eyes" is more likely to be directed toward males and females who happen to wear glasses rather than those who make use of contact lenses, since the glasses serve as a readily visible sign of the need for some sort of vision correction devices. While generally considered more of a playground insult, the expression is sometimes used in adult settings.
The imagery begin "four-eyes" has to do with the idea that an individual who is in need of corrective lenses is somehow deficient physically. Since the eyes are not capable of perfect vision, each eye must be assisted by what amounts to a second artificial device or eye in order to focus properly. With this in mind, the phrase is taking into account both of the natural eyes of the individual, plus each of the two corrective lenses that are used to allow the individual to see more clearly.
Since the term is normally used as a means of derision for the individual wearing glasses, it is rarely utilized except in situations where someone wants to express contempt or dislike for the person who is wearing the glasses. The implications that come with the use of "four-eyes" include the perception that the wearer is physically weak, unlikely to engage in any activity that requires much in the way of physical exertion, and will abstain from any activities that could lead to damaging the lenses. In reality, people who wear glasses and other types of corrective lenses engage in a wide range of activities and are often in robust physical health.
In most quarters, to use the phrase to describe another human being is considered to be a sign of poor social skills, and a level of ignorance that buys into the idea that anyone who wears glasses is weak or deficient in some manner that prevents them from enjoying life and participating in whatever activities they choose. While it is true that people who actually wear glasses may sometimes refer to themselves as "four-eyes" while playfully engaging in conversation with others who also wear corrective lenses, the context is very different from the more common application of this idiom.