That picture looks pretty good for a belly on a new hatch. Do you have a more recent picture?
I'd try putting her with mum if she's walking, but under very close supervision. Mum may not recognise her chick and if she doesn't accept her, she'll probably attack the "threat" to her own brood's food. A broody can and often will kill a foreign chick. The longer the chick is away from Mum, the less likely Mum is to accept her.
If Mum does accept her, they'll all need to be penned in a small space with food and water until the late hatcher can keep up with the others. Mum hens lead the chicks. If one can't keep up, that chick is a weakling and an acceptable loss. That's just how chickens are.
If Mum doesn't accept her, you'll need a brooder and at least one other chick for a companion. You can take one of the other three, or, if that makes you feel guilty, see if a local feed store is carrying chicks. Chickens are flock animals and do not do well on their own.