I've successfully downgraded my phone to ZV3, towelrooted, installed CWM recovery, flashed donjuro's ZV4 ROM, flashed boot and kernel (whew) so I believe I'm successfully rooted and on ZV4. But (you knew there was a but) the Superuser which came with the ZV4 ROM says that the binary must be updated. It asks to chose an installation method and the only choices are "Cancel" and "Recovery Install". Recovery install give an "untrusted zip" warning. Even if I ignore the warning it does not upgrade Superuser. The Superuser is verison 184.108.40.206 by Koushik Dutta. I have installed a different Superuser from the Playstore which does work. There doesn't appear to be a way to remove the Koushik Dutta Superuser.