I think that 4 weeks is waaaay to young to purchase a pup,if you take a pup this young home,it will deffinately bond with you,but it wont be a very social dog,especially with other dogs.Most breeders have just started weaning their pups at that age,so taking a pup straight from its motherīs teet,into its new home just seems cruel to me.
At 4 weeks you have no idea what kind of personality this puppy will have,it wont be eating properly yet at this age and its still learning from its mommy and siblings.
Its selfish to just worry about the bonding..I think its alot better to buy a healthy 8week old puppy and bond with it later on.Once you have your healthy puppy at home,you have all the time in the world to bond with it..theres no rush