The 4 days in transport probably did them in.
The ones that survived 4 weeks should have continued to improve if their nutrition and ventilation was optimal.
Are you trying the bread, fruit and vegetables to encourage eating?
Those things aren't really appropriate for chick nutrition. You really can't improve on non-medicated chick starter since it's formulated with all the vitamins, minerals, energy, fats and amino acids in the right ratios chicks are known to require.
The other things are low in protein and the bread is too high in salt.
Ill weak chicks aren't likely to go to the effort of eating fruit and veggies.
If they won't eat on their own, they may need tube feeding. Make sure they're well hydrated before you try to feed them.
Put some poly-vi-sol without iron in their water, or a drop directly in the mouth.
Make sure the brooder has a warm spot but not too hot.
Try mixing some warm water with the starter to make a moist mash.
A probiotic is advised as well.
Since you're in CA, you can get a necropsy free. It would be good to send one to the state poultry lab to make sure it isn't a problem other than the degraded vigor from long shipping time.