Seriously, I have no clue how she fit them all inside her, but there were definitely four of them mewing under the blackberry bushes today when I went to check. Observe:
It's still going to be precarious for a while yet (at least 2 weeks) given how tiny these larval-stage kitties are and how small their mom is but I am more optimistic now than I was last night. At the very least, Bella has definitely cleaned off the babies quite nicely (in the picture you can see how soft and fluffy their fur is), and they don't seem to have crusty eyes or anything. And they (and Mom) are eating a ton, so that's promising too.
We could have easily picked the kittens up today and brought them into the house (with the idea being that Bella would follow and live mostly indoors until the family could be taken in for spay/neuter). But between me and Matt and his mom the consensus was unanimous that we should probably leave well enough alone for now. Bella is relatively bold for a feral cat but she is still far from being socialized to humans and we would not want to breach the trust she seems to be gaining in us. And as long as she is taking care of her babies it is best that she be able to do so with minimal stress and I think she'd have a pink fit if any primates touched her offspring at this point.