They start to show their personalities at 4 weeks (mostly dominance games), but until 6 weeks you really can't tell that much about them.
At 7 weeks, I do the Campbell Test on my puppies (it is a personality test). Google it and you will find it easily. It's fun to do too!!
I do not show my puppies or accept deposit on them until reach 8 weeks of age. I feel like before that, I know to little to properly match them to the proper family.