Xiao and Venti are only acquaintances and Kazuha hasn't met any of them besides Aether. This makes Aether the only one friends with everyone. Despite this, they do have some similarities. All four of them are boys that are playable characters that can use Anemo, though Venti has a gnosis and Aether is without a vision. They have all lost someone close to them. Aether lost his sister, Kazuha lost his good friend, Venti lost his close friend that he has taken the form of and Xiao lost many people due to his chronological age being 2000+.
The four are all from different regions. Venti is from Mondstadt. Xiao is from Liyue. Kazuha is from Inazuma. Aether is a traveler. It would be unlikely for all four of them to be gathered in one place, even if they are really similar.
Some of them look alike as well. Xiao and Venti have similar color schemes with green and blue. Kazuha has platinum blond hair in a ponytail while Aether has bright blond hair in a braid. Aether and Xiao both have golden eyes. When it comes to their attacks, there are very few similarities if you rule out their shared ability of Anemo. Kazuha and Aether are the only sword users of the group and Kazuha can use his elemental skill in the air like Xiao.
The ship polyship became more popular because of AUs where they're in a band called 4NEMO. The name comes is 4NEMO because there are four of them and all four of them can use Anemo. In idol and band AUs, there is also a group with Childe, Zhongli, Kaeya and Diluc that is sometimes shown alongside 4NEMO. There's also a group with Beidou, Ganyu, Keqing and Ningguang as well as a group with Bennett, Chongyun, Razor and Xingqiu. While 4NEMO is normally portrayed as a boyband, Lumine is sometimes portrayed covering for Aether.
With the release of Shikanoin Heizou, many have decided to add him to 4NEMO, creating "5NEMO," "AN5MO" or "5WIRL." Others have decided to replace Aether with Heizou since Aether isn't always Anemo. Some have also noticed that Heizou, Kazuha, Xiao and Venti all have hair highlights while Aether doesn't.
A lot of people view the 4NEMO as a friendship, though those who ship it romantically tend to see it as a double date since there are four of them. The most common way to view it is a double date between XiaoVen and Kazuther. Though, there are those who see it as a double date between KazuXiao and Venther or KazuVen and Xiaother.
The one who started this popular idol AU was an artist named Veechu (veechu_ on twitter), however the popular AU was somewhat abandoned by the fandom due to strong controversy.
- 5WIRL refers to the ship between the four and Shikanoin Heizou
- Kazuther refers to the ship between Kazuha and Aether
- KazuVen refers to the ship between Kazuha and Venti
- KazuXiao refers to the ship between Kazuha and Xiao
- KazuXiaoVen refers to the ship between Kazuha, Xiao and Venti
- Venther refers to the ship between Venti and Aether
- Xiaother refers to the ship between Xiao and Aether
- XiaoVen refers to the ship between Xiao and Venti