Hi @Alastair0
It's so frustrating when this happens isn't it?
the overall rating is not an average of all the other stars, it's what the guest chooses to allocate it unfortunately. And the system is really more or less designed to suggest to guests 4 is perfectly ok.
There are many many threads re this on these boards.
Some host specifically educate guests about the damage anything less than a 5 can do - it's not the guests fault - in a real world 4 is a really good score - they no doubt think they are being generous.
The prompting guests if the scores dont' quite match I think only kicks in for 3 stars or below.
I can send you a link to what some other hosts say in their manuals for guests re the scoring system if you are interested.
Regards