NetHack 3.4 as there was no NetHack 3.5 release.
NetHack 3.6.4 is the official release of NetHack that follows NetHack 3.6.3
-and contains all bug fixes since then, including one security-related fix.
+and contains all bug fixes since then, including one security-related
+fix for a vulnerability potentially impacting 3.6.0, 3.6.1, 3.6.2, and 3.6.3.
The file doc/fixes36.4 in the source distribution has a full list of them. The
text in there was written for the development team's own use and is provided
"as is", so please do not ask us to further explain the entries in that file.