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The DevTeam released nethack-3.4.3 in Dec 2003, and then pretty much went dormant for 11 years -- no new releases, no roadmap, nothing. So some time around 2013 some people not connected to the DevTeam created this project: http://nethack4.org/ . So now the DevTeam skips 4.x version numbers completely to avoid confusion.

man that's super shady that they didn't rename the split if they didn't have buy in from the original devs to take over the project.

The people who worked on NetHack4 are now members of the mainline DevTeam and were heavily involved in the 5.0 release.

Nethack4 is the name of a popular fork, they probably wanted to avoid confusing people

They likely were inspired by Microsoft's innovation in skipping 4.x of DirectX.