I wish the pictures were a little bigger! However, judging from the "spread eagle" stance in the first picture, I'd say you do have a quite young joey - and one that shouldn't have been parted from her mother.
If he is eating anything on her own- then you don't have to feed Suz's diet. Baby cereal isn't really a good option for joeys. Instead - try a variety of baby foods. Baby food chicken is a good start - she'll need the protein. Then add some baby food peas and green beans, some applesauce, some carrots. Make sure the baby foods do not include added salt or other seasonings. Onion and garlic are toxic.
Does she have a sleeping pouch? If she has a fleece sleeping pouch and you give her plenty of fleece blankets, that should help her. Additionally, you can wrap her in fleece and carry her down your shirt for warmth. Do not sleep with her in your shirt, however.
Anything you substitute can NOT have artificial sweeteners. That's why Gatoraide is discouraged ... it usually does have sweeteners.
Do you have a space heater you can put near her cage (not too near, of course!) at night? At 4 weeks old, she isn't really very good at monitoring her own body heat.
Make sure she stays hydrated. Make sure you see her drinking, and see if you can offer her some water or diluted fruit juice several times a day with an eye dropper. Fluids are important to her. Don't force her to drink - because she could choke and aspirate the fluids into her lungs. But do wake her up several times a day (maybe every 3-4 hours?) to encourage her to drink.
Good luck! She's adorable!