Sparknotes summarizes 1984’s Chapter 4 this way:
Winston goes to his job in the Records section of the Ministry of Truth, where he works with a “speakwrite” (a machine that types as he dictates into it) and destroys obsolete documents. He updates Big Brother’s orders and Party records so that they match new developments—Big Brother can never be wrong. Even when the citizens of Airstrip One are forced to live with less food, they are told that they are being given more than ever and, by and large, they believe it. This day, Winston must alter the record of a speech made in December 1983, which referred to Comrade Withers, one of Big Brother’s former officials who has since been vaporized. Since Comrade Withers was executed as an enemy of the Party, it is unacceptable to have a document on file praising him as a loyal Party member.
Winston invents a person named Comrade Ogilvy and substitutes him for Comrade Withers in the records. Comrade Ogilvy, though a product of Winston’s imagination, is an ideal Party man, opposed to sex and suspicious of everyone. Comrade Withers has become an “unperson:” he has ceased to exist. Watching a man named Comrade Tillotson in the cubicle across the way, Winston reflects on the activity in the Ministry of Truth, where thousands of workers correct the flow of history to make it match party ideology, and churn out endless drivel—even pornography—to pacify the brutally destitute proletariat.