During the fight between Jiraiya and Tsunade against Orochimaru and Kabuto in the Search for Tsunade Arc, Jiraiya refers to Kabuto as "Four Eyes". Why did he get that name?
Because he was the only one among all present there that was wearing a pair of glasses.
The term "Four eyes" refers to people that wear spectacles.
I don't recall the scene. But in Japanese, he probably actually referred to him as megane (meaning spectacles). Rather than using literal translations, subbers/dubbers prefer to use equivalent terms which will carry a similar tone in the target language. Since an equivalent epithet used in English for a bespectacled person is four-eyes, that is what was preferred.
Four-eyes is almost always used as a derogatory term in English. But this isn't necessarily the case with megane. That said, considering the character of Kabuto, I expect that that is how it was used in this case. IOW, four-eyes is a perfect fit.