That kitty is lucky to have you around!
How long has it been since she delivered the kittens?
Does she let you pet her? Are you able to check her nipples to see if any milk comes out or if there is some swelling?
Kittens are very quiet and sleep a lot. At 4 days old they aren't going to be moving around much, either. I suspect mom has the litter tucked under there and they are all doing well.
Definitely keep feeding her kitten food and water (she doesn't need milk if she is eating a good quality kitten food).
You can gently squeeze one of the bigger nipples and see if a drop of milk forms. If so, then she is definitely nursing. It is possible, too, that she only had one kitten.
I would leave them be. If you try to disturb them, mom may move them elsewhere.
They will start getting more active around 4 weeks of age and you might start to see them at that time. If mom is friendly with you and trusts you, she will let you see them when they are ready.
Gentle squeezing shouldn't hurt her--the kittens suck pretty hard! :)
She might. If the area under the wall is warm, dry, and safe, she may leave them there, too. But once they get older and start moving around, she may be more interested in moving to the box you've provided.
The kittens really don't need to be seen by a vet until 6-7 weeks of age. At that point, you should be able to catch them and mom to get them to a vet. They should be tested for feline leukemia, dewormed, and given their first vaccinations. At that point they can also be put up for adoption.
Feed a good quality kitten food--stick with a brand name that doesn't contain lots of colored bits. Canned is good as there is more moisture. When the kittens are 4-5 weeks of age, they may start eating the canned food, too.
I don't think so. If all the kits were dead, she wouldn't be producing much milk anymore and I really don't think she would be staying with them. They will kick sick ones out of the nest, so if they were all sickly I don't think she would stick around at all.
You are very welcome!