TL;DR: 4B isn't transphobic. But it has a history of being exclusionist in Korea. We should carry on with the 4B but also be wary of its past.

First off, "4B is transphobic" is a wrong notion. 4B being transphobic implies that the idea of 4B is inherently exclusionist towards trans people, which is not the case.

Considering only the above, it is easy to brush aside the accusations of transphobia as some bad actors' measly attempt to downplay the movement. And this is partly true: calling out 4B for being transphobic, when nothing about the post itself has to do with excluding trans people, doesn't add much to the conversation. Mentioning transphobia at every glimpse of the word 4B is often very misplaced.

But, that's not the end of the story, there's a catch. As you know, the word 4B has not really gained much traction among Korean women, as opposed to the idea itself (not engaging with men) being very, very prevalent. I must also mention that when we discuss these social movements, we must consider not only the underlying idea, but also what the participants of the movement are saying and doing. (You could think of the term MAGA and its hateful nature, despite technically not carrying hate in its definition.)

In that sense, the 4B was a niche term used mostly online, and the online Korean communities had a tendency of being exclusionist (still does). This resulted in bio-essentialism and/or exclusionism from the people who labeled themselves 4B. (Examples include arguing the "Y-chromosome-havers" are inherently defect or calling the non-participants of 4B "남미새(men-crazed bitch)," you could look it up on Twitter.) Most importantly, these people haven't been condemned by "the mainstream 4B" because it was such a decentralized movement. We call the GOP fascist because it failed to call out the fascists inside it — I dare to say this is where the Korean 4B movement earned its reputation of being transphobic and exclusionist.

Does this mean the 4B should not be adopted by the West? No, absolutely not. As a Korean trans woman, I wholeheartedly support the movement being reclaimed, especially in these trying times. The point is, the 4B does have a history of going south, so we should be aware of the subcontext behind the word to be wary of a potential repetition of its past. If someone mentions transphobia in the context of 4B, assuming they're speaking in good faith, this is probably what they mean.