OP - I am with your friends on this one.
Schools like KP are good in picking girls that have a good academic potential. So - your daughter must be mature and ready to learn. And best for her will be to learn at the speed that works for her.
In a school where she’ll be among only a few similar girls - that might or might not happen.
I had that with one of my DD - she was at a good school, but ahead in a few areas and they just couldn’t or wouldn’t challenge her appropriately. So - at times they’ll tell her to sit and read while others completed their tasks.
And the problem with that for me was Not that she wasn’t learning more advanced material - but rather that she was used to everything being easy and wasn’t learning to make an effort and work.
So - my worry was that when she gets to secondary and work picks up - and things would stop being easy - she’d not have learned to work.
So I moved her at 7+ to a school that had more girls like her. And things stopped being easy all the time - but she was so much more interested and happy.
Thing is - your feeling about one school vs another - at this point based on a one (few?) visits. And that is just a Polaroid, one day in a school’s life.
Also - your 4yo will grow and change a lot between now and Y6.
I have friends in both of these schools. And like with any school - you can find a range of opinions.
KP is a great school for a bright child. The girls are bright and confident. And it has facilities to match.
And is a bit more down to earth than GH. Which seems quite precious.
Another point of reference is Bute 7+.... Most schools in the area have their brightest girls try to love there at 7+. Garden house certainly loses a few girls/year. With more trying and not making it.
While KP doesn’t have this issue.
To me - this is one of the best indicator on whether people are happy there.
And I won’t even mention the dreaded 11+. And peace of mind that comes with knowing that your school would do as best for your child. Vs being stressed and having to do a lot of the work yourself because your school can’t do that.
At 4 you think it’s far away.
But it isn’t.
Good luck.