I'm not a pilot or an expert on FTL but would have thought that with four crew and good crew rest facilities a reasonable rest period of eight hours was shorter than the maximum duty time for an un-augmented crew so four crew could cope with a duty time in excess of twenty hours. That's just an observation on my part pilots may have their own views!
In a relatively short while, the current political debate on European FTL will reach a conclusion - one way or another - the file will be closed and the various protagonists will retire to count their bruises and lick their wounds. This particular dossier is not going to be reopened any time soon.
Yes, of course, 20-hour sectors could be operated with augmented crew. But not with four pilots, my guess is that the airlines would say five, the unions would say six. Either way, that's a lot of capacity to set aside for bunk rest and a lot of hotel bills at the other end. But that's not the point. Current rules do not foresee this eventuality, nor can they be interpreted to accommodate it. it would require the rulebook to be reopened and revised, and the whole circus starts all over again. I really don't believe that's going to happen, for the sake of a business propostiion which is questionable at best.