Where on Earth is this setting inside the Calibre preferences menu?? I just spent about 5 minutes looking and simply couldn't find it anywhere!
I also checked that the destructive edit happens upon viewer close, not open or page turn.
Also the version 4 ebook viewer is step back from the old one, as it stands now. It is slower, more resource hungry and buggier. The only arguable improvement is it now uses a 'flat' theme that doesn't look dated. I'm even considering reverting back to Calibre 3 because of the ebook viewer randomly freezing on large epubs with equations and so on.