While I can't speak for that particular rescue, could it be possible that the difference in fees might have to do with how much the rescue has had to put in to the dog? Maybe the 2 year old needed to be spayed and the 4 year old came to them already spayed. Just an idea.
And perhaps the difference could also be a way of encouraging the adoption of a slightly older dog because the pups and the young ones go to homes much faster.
As for the health, no a vet check doesn't guarantee a problem free dog. But then neither does a health guarantee from a reputable breeder. Nothing in those contracts protect you from illnesses down the road that aren't congenital or injuries or allergies, GI problems, etc. We all take our chances when we bring a dog in to our lives.
__________________  Don't get your knickers in a knot. Nothing is solved and it just makes you walk funny. |