Comment by robrtsql

They cover that in the FAQ: http://nethack4.org/faq.html

Basically, NetHack development stalled out, so the developer(s?) behind NetHack 4 decided to fork it. Unlike other forks whose main goal is to put cool stuff in the game without regard for taste or balance, they wanted to stay true to the original.

Not that I agree with the fork name. It's best to not call your fork something that could be confused with the original, were the original devs to ever reach a 4.0 release.