Lagatta is correct that a kitten this young should not go much longer without food and water (or some form of liquid) or dehydration is the result. I would try giving her goat’s milk or kitten formula mixed with her wet food. Of course, she probably knows that she is in a different place, but with plenty of attention and love she will get over that.
If you trust that the foster is telling you the truth, then she should have no trouble eating once she is hungry enough. A lot of times I have found that they are not truthful and the kitten is barely weaned or just starting to be weaned. Usually kittens aren’t fully weaned until 8 to 10 weeks. Some do well sooner than that. If you provide a litter box with plain clay unscented non-clumping litter, and place her in it after she eats, then she will soon figure it out. You could also stimulate her to pee and poo over the litter box and then when you place her in it, she will smell it and start to use it on her own. Each kitten is individual and learns at his/her own pace, so just be persistent.
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