Try not to worry too much.
I've had to hand rear a couple of litters from birth in the past.
Don't forget that you'll need to keep her warm, the most important thing, & clean as well with no mum to do it.
Feed her four small meals a day. At that age I feed two meat meals & two cereal, make sure everything is mush or chopped small, & not too hot but not stone cold either. Can you get a bitch milk substitute? I use Lactol but not sure if that's available over there. Mix it with puppy porridge, baby porridge will substitute if you can't get it.
After she's eaten clean her face & make sure she wees, you'll have to clean her bum after she goes, then give her a wrapped hot water bottle to snuggle up to.
She should not have been sold so young but she should be fine if you spend time looking after her. Being a small breed means you need to be a bit more vigilant.
Have to say, with my other slightly bigger breed, some of the mums have been sick to death of the puppies at four weeks, & wanted to get away as much as possible, while the puppies have been doing their utmost to eat everything I feed their mum!
So try not to worry too much & enjoy her! I bet she'll be particularly tied & loving to you. Of the puppies I kept over the years the hand reared ones were always the extra special ones.