Well, we took Zoë home at 11.5 weeks. I'd seen pictures of her since she was just a few weeks old. We made a trip to visit and "picked her" when she was 6 weeks old. She had just started to have her own personality, but the puppy we saw at 6 weeks was VERY different than the puppy we took home, still adorable and still the best puppy ever.
I think the breeder is doing her best for you, at 4 weeks old they aren't old enough to tell much of a difference and which would be best for you. 6 weeks may be a bit too young, but once you get to 8, 10, 12 weeks then you can for sure tell differences in personalities. It does vary though, there are people that never see their dog before taking it home (so they would pick their puppy at 12-18 weeks).
Best of luck! I'm sure you'll find the most perfect puppy for you!
-Vicky & Zoë