There has never been a version 4.0. If you went all the way back to 4.0.0 and it still didn't work then it can't be a hardening problem, since hardening protection wasn't introduced until 4.3.14. Unless you had an entirely different problem that is, because I'm not sure 8.1 was a supported host for 4.0.0 (in fact I'm quite certain it wasn't), so naturally I wouldn't expect it to work.
What AV software are you using? Have you installed a hacked theme?