That is not a breeder! That is a money-hugging bitch owner with not much sense or feeling of responsibility!
4 weeks is when the most important socialization period in a dogs life begins, at 12 weeks of age it starts to decrease and at 16 weeks, the period is over. A dog can obviously still learn to socialize more or less well with other dogs after this period but not at the same speed, it is more difficult etc.
If you take a puppy at 4 weeks of age and you don't already have a vaccinated dog that your puppy can socialize with, you will have to wait until the puppy has finished his/hers puppy vaccinations at 12 weeks of age, before you can allow it to socialize with other dogs and then the "big socialization window" is already closing!
Will it be vaccinated and have had a puppy vet check? Actually I don't know if there is vets that will vaccinate puppies already from 3, 4 weeks of age?
Will the bitch owner take back the puppy if it doesn't work out? If not, they're sponsoring a person to continue with his/hers money-hugging business/"hobby". And if it doesn't work out and the bitch owner doesn't take back, what happens to the puppy then?
We found and rescued an abandoned, half starved and half frozen to death, about 4 weeks old kitten last autumn. I was home all day, my three dogs helped me "entertain" it (our two cats didn't want anything to do with it) and I can only say that I would never pay for a 4 weeks old puppy, I would quite the opposite expect the breeder to pay me something for taking over their work in half time!
I was just asked what I was writing about. I said someone wanted reasons for why someone they knew, shouldn't buy a 4 weeks old puppy.
"What's wrong with buying a puppy? - - - - - - WHAT, how old did you say? 4 WEEKS, OH MY DEAR GOD! That is way to young, poor puppy..."