Crate training is definitely a good thing! And for sure let the other dogs discipline within reason, with other dogs around I am betting she will turn out ok.
At almost 4 weeks give her lots of noisy toys and obstacles to deal with, make her think and every once in a while, partially block her way with stuff so she has to either go around or climb over. Bang pots and pans together or drop them near her, at this age fear hasn't developed yet so noises at this age they recover quickly and learn to ignore loud sounds. My gun dog friends say this is the perfect age to get them used to the sound of gunfire, if your family does any kind of shooting.
You can even begin basic training at this age, my sheltie pups were taught to sit and shake at 4 weeks old.
Also house training can begin at this age, after eating, drinking and napping take her out!
I would also use a stuffed toy (not your hand) to imitate other puppies in the litter, IE gently knock her around a bit and rough house but if she gets to rough yelp and remove the toy for a bit.
That's all I can think of for now, if you can get the book I like "Another Piece Of The Puzzle" by Pat Hasting it has lots of good tips for puppies from birthand up, and even a section on singletons.