Files are in the usual place
. There is also a patched 3.5.1 version that just fixes the Draconian Polymorph bug (sorry about that).
4.0.0 should be considered beta, as any X.0.0 release. It also breaks savefiles, so you'll need to finish up your current characters on 3.5.0 or 3.5.1. before upgrading.
There have been many, many changes for this release, almost 400 commits in fact. But most changes have focused on the Help files and various UI improvements (For example, there are window options for the Monster and Object Lists).
I apologize in advance for any bugs ... I changed a lot of code for this release. I'll try to do better at keeping up with bug reports, but without internet access, this is difficult.