Since it was clear the momma cat was not very effective, the foster mom brought home the momma she had previously, Mila. Mila instantly went to work cleaning the kittens while the other mom nurses them. Then Mila started bathing the momma, too!
We are hoping for the best for the remaining two, but understand that fading kitten syndrome is quite common in young mothers, who often pass viruses and ailments to their kittens that older cats with stronger immune systems do not carry.
Update: We are down to just one kitten from four, but Mila (right) is not leaving the side of the other mommy and the baby. These two mommies seem happy to help each other and keep each other company. A ginger alliance to keep the remaining baby warm, fed, and well cleaned.