We've just bought a new pony for our 7 yo DD - she is also tall for her age. We bought a 12.1hh pony for her. She's been out pony racing on our 11.1hh this spring, she was the youngest in her race. The others were up to 10 years old, and not one of them was on anything over 12.2hh. A 14hh Connemara is a performance animal much more suitable for a teenager, definitely not for a 4 year old on the lead rein. I appreciate that you don't want to sell a pony on when it is outgrown, but really - ponies are happier and healthier when they are kept in work suitable for their breed, size and mentality, and often that is more important to them than being in a 'forever home.' The sentimentality is always the human's in these situations, not the pony's.
So you need a pony that will not go nuts when all it does is trundle up the road for 20 minutes at a slow walk. Thing of it in the context of cars - your child may one day aspire to own a top of the range sports car, but you wouldn't send them out for driving lessons in a TVR or Porsche, you would probably find them something like a Toyota Yaris or Ford Focus. People who jump in a performance car with no experience and think they are capable of driving them usually end up wrapped round trees. By putting your daughter on a pony she does not have the knowledge, ability or muscle strength for you will be inviting a loss of confidence at some point at the very least, or an accident at the worst.
If you want something you can ride that she will grow into, buy yourself something nice and safe and ploddy, get her a proper lead rein pony on loan, and ride out together. That's what I do with my 4 yo - and I expect him to stay on the 11.1hh for at least another 4 years! (And yes, he is tall for his age too.) We'll have had the 11.1 for 8 years by that point, and then he will go on loan (because I don't want to sell him, and he will be old by that time. But will still be healthier for being gently hacked about and teaching another young child to ride.) But please don't buy a 14hh and think your daughter will be able to ride it independently, competently and safely at any point in the next 6 years! I see over horsed kids at Pony Club all the time, being hauled about by a pony too big for them (and I have bought that Tshirt too - I over horsed my then 6 yo with a 12.3hh - far too much pony for her ability and needs at that time.)
Back to the first point you were discussing though - there are some complete twats selling ponies out there. I've made appointments numerous times only to have to disappoint my daughter when someone has texted me the night before a viewing to tell me they've either changed their mind or sold the pony to someone else. That kind of person needs removing from the equine market place. But there are lovely sellers out there too. They are the ones I've been lucky enough to buy from - I keep in touch with all the sellers of horses and ponies I've bought recently. Keep looking, and you'll find something that suits you.