In this case, there's no "must have" as far as the union are concerend, it's more that the company have totally disregarded the agreement that's already in place, and are trying to impose the new roster without agreement from the people affected.
No one has said it must be 4 on 4 off, not even close to that yet, but what is not at all easy to cope with is 17 days with only 2 days off, and one of them is after a 14-22, with an 04 start after the day off, so that means it's only a 30 hour break overall, and there's 2 sleep periods of in theory 8 hours to come out of that time. It's bad enough making a change from late to early, (or the other way round ) when there's 2 days off, even worse when it's only one.