4B or "Four Nos" is a radical feminist movement that originated in South Korea. The name refers to its defining four tenets which all start with the Korean-language term bi (Korean: 비; Hanja: 非), roughly meaning "no". Its proponents do not date men, marry men, have sex with men, or have children with men. The movement emerged between 2017 and 2019 on Twitter and on the website WOMAD. It has since spread internationally, namely to the United States after its 2024 presidential election.

The movement is considered fringe in South Korea, and has since declined. Estimates of the movement's adherents vary; reportedly several articles claimed around 5,000 and one article claimed 50,000. In South Korea, a portion of its members, particularly those associated with the openly misandric WOMAD, were described as transphobic and homophobic towards gay men.

The four core tenets to the 4B movement are:

• no sex with men (Korean: 비섹스; RR: bisekseu),
• no giving birth (비출산; bichulsan),
• no dating men (비연애; biyeonae), and
• no marriage with men (비혼; bihon).

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A 2022 survey of unmarried South Koreans aged 19 to 34 found that 69.7% of women (compared to 79.8% of men) expressed a desire to marry in the future. Marital violence is the most prevalent form of family violence in South Korea. In heterosexual South Korean households, women perform a disproportionate portion of the unpaid labor. Some 4B followers state that the movement helps protect them from the risks of marriage, including domestic violence and an unequal distribution of labor.

Many South Korean women are reluctant to have children due to the lack of workplace accommodations. More than 40 per cent of South Korean women take an extended career break after marriage and childbirth, while many of those who stay in work struggle to progress their careers. The 4B movement proposes that women focus on financial independence, including forgoing childbirth.

South Korea has one of the lowest birth rates in the world. As of 2023, the fertility rate is at 0.72, significantly below the 2.1 threshold required to maintain a country's population. The country's birth rate has been below the replacement rate since 1983, while the 4b movement originated in the 2010s, making it likely that the low birth rate is due to economic insecurity experienced by young adults, high child-rearing costs, high property prices, and the double burden placed on working mothers.

A 2022 survey of unmarried South Koreans aged 19 to 34 found that 55.3% of women (as compared to 70.5% of men) indicated a preference for having children.

Women of the 4B movement do not engage in romance and sexual relationships with men, because they see it as an extension of the patriarchal family structure.

Jung Se-young and Baeck Ha-na, two proponents, criticize marriage as reinforcing gender roles in South Korea. The movement draws some inspiration from the novel Kim Ji-young, Born 1982, as do South Korea's MeToo and "Escape the Corset" movements.

The term 4B emerged from Korean feminist circles on Twitter around 2017 to 2018, after a highly publicized 2016 murder of a woman by a man. The murderer, who said he did it because women had ignored him, was not charged with a hate crime.

The 4B movement also emerged in response to many other social issues. For instance, the misogynistic social media platform Ilbe Storehouse grew in prominence in 2014, and was followed by cultural "gender wars" in 2015.

The 4B movement gained broader recognition on Twitter in 2019 and through various feminist social media accounts. One notable feature of the 4B movement, as with other Korean digital feminist movements, is that members often identify themselves as "anonymous women," as it is conventional not to disclose personal details online.

The "Escape the Corset" Movement that started in 2016 served as a source of inspiration for the 4B movement. The movement calls for women to liberate themselves from sexual, social, bodily, and psychological oppression. The word "corset" is used by Korean feminists as a metaphor for the societal mechanisms that bind and repress women, including toxic beauty standards. Notably, South Korea has the 10th largest beauty market globally and is the third-largest cosmetics exporter. In a society where beauty holds immense cultural and economic significance, members of the "Escape the Corset" Movement criticize and resist cosmetic procedures, demanding skincare or makeup rituals, and the adoption of trendy clothing, all seen as perpetuating consumerism and misogynistic social norms. In protest, they express their defiance by destroying makeup, forgoing cosmetic enhancements, shaving their heads, and rejecting fashionable attire. Escape the Corset's analysis and approach to protest deeply influenced the 4B movement.

Although the #MeToo movement originated in the United States in 2006 and gained popularity in 2017, many other countries, including South Korea, created #MeToo movements of their own. The #MeToo movement in South Korea, like those in other countries, encouraged women to express their experiences of sexual harassment to inspire social change. Shortly after its inception in late 2017, several hundred women stepped forward with claims of sexual harassment and violence. This movement also encouraged previous comfort women of the Imperial Japanese Army during World War II and the Japanese occupation of Korea to speak more freely in large numbers about their sexual abuse.

The #MeToo movement also inspired various online hashtag campaigns, most popularly the #WithYou tag, to signal solidarity with survivors of sexual assault who had spoken up in the #MeToo movement. These various hashtags inspired the formation of women's activist groups, such as Citizens Action to Support the #MeToo Movement, which campaigned to end gendered oppression and support victims of sexual abuse in South Korea.

In South Korea, the movement has lost momentum since its early days.

Although the exact membership remains uncertain, some unverified estimates suggest a range of 500 to 4,000 claimed participants.

After the 2024 United States presidential election in which Donald Trump won a second term, some American women expressed interest in the 4B movement as a form of protest against Trump's election, his alleged sexual assaults, and his role in the overturning of Roe v. Wade. Other American women expressed interest in 4B as a method to support other women and to protect their health and safety should they lose access to birth control or abortion. Shortly after the election was called, TikTok videos mentioning 4B were viewed hundreds of thousands of times, and Google searches about it spiked by 450%. American women have called the movement the "4 Nos" and "Lysistrata".

The 4B movement is considered to be fringe in South Korea, with Ju Hui Judy Han of UCLA arguing that "the vast majority of South Korean feminists do not abide by it," and that "4B is not representative of Korean feminist politics." One woman interviewed expressed frustration with the Western adoption of the movement, claiming that the conditions for the movement in South Korea significantly differ from those in the West. Internationally, various hashtags and social media posts have been used to express support for the movement.

Radical feminism as a whole in South Korea has had a notable transphobic and homophobic (against male homosexuals) presence, with internal dispute about the acceptability of such beliefs. The 4B movement was significantly popular on (and widely publicly associated with) the South Korean website WOMAD, which is considered to be misandric, homophobic, and transphobic. The website was founded because Megalia had begun prohibiting homophobic and transphobic slurs. WOMAD members reportedly advocated for revenge against men, advocated for disliked people to commit suicide, and some threatened violence and committed crimes against men. Mothers were labeled both victims and conformers to patriarchal society that discriminate against their daughters. Many WOMAD members compared married women to slaves. One trans person interviewed in South Korea argued that, while most South Korean women were not transphobic, the strongest advocates for 4B in South Korea were on and radicalized by WOMAD.

Some members of the 4B movement have reportedly advocated for the use of ID scanners to verify sex before entering public restrooms, and for more sex segregation at protests. Various communities involved in the 4B movement reportedly asked that members take photos so that others could verify their biological sex; one such group asked for videos of people's Adam's apples.

Some South Korean queer and trans feminists reportedly expressed concern that the 2024 international interest in the 4B movement could lead to an increase in anti-trans rhetoric and a resurgence of the 4B movement in South Korea.

• 6B4T movement
• Domestic violence in South Korea
• Female separatism
• Gender inequality in South Korea
• Men Going Their Own Way, antifeminist movement among men advocating disengagement from women
• Neijuan
• Political lesbianism
  • Queer nationalism
• Anti-natalism
  • Shakers, an 18th-century religious movement whose members also refused marriage, sex and child-rearing against the wider culture; they eventually went extinct
• Sampo generation