I had this issue when mum was in hospital last summer for 4 weeks. Because it was exactly 4 weeks, I didn't advise DWP. SS got involved straight away as mum had a stroke which took her mobility away, so the care package needed readdressing anyway. I think they said that the existing care package would stay in place and that any surplus could be addressed later. As it turned out, they gave us a temporary increase after mum was discharged and the 'surplus' got mixed up in all of this and other extra care I had at the time, so in effect no change.
As always, I suspect the rules vary from LA to LA. I think if you inform SS, then that's all you would need to do as the finance people are, I assume, part of the same team.
I believe the 28 day rule applies to all.