You have a baby that is too young to be away from its dam and litter. In most states it is illegal to sell them this early. It can lead to problems, particularly in dog-dog interaction and bite inhibition. A four week old puppy is barely weened if it is. Actually mine turned four weeks old on Thursday and I just started introducing food on Wednesday. Just started introducing water today. The pup needs its momma. And if the momma died, it needs its littermates.
If the puppy is alone in the world, then you just have to compensate. Sometimes there are orphans, but we should never make an orphan out of a puppy if it is not one.
That coloring, I think is pretty normal. It is hard to be sure with cameras. It might come out as black and tan. And should be a saddle back in markings.
If you can return the puppy to the breeder for another month, that would be ideal. If you can't then don't take it to pet stores. When you go to the vet, do not set the puppy down. It does not have immunity from diseases, it can barely support a fever, and it has to be kept warm, but not dehydrated.
Bear Cub, Hepsi-Pepsi
Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.