The main thing you need to do with kits is to make sure that they're all being fed by mom. She'll only nurse them 1-2 times a day for about 5 minutes (usually late night/early morning), as rabbits' milk is extremely rich and in the wild, mother rabbits stay far away from the nest as much as possible to avoid luring predators back to it. Don't expect to see her sitting on the nest!
Every morning, make sure all of the kits are warm and have plump, round bellies. If any of them don't seem like their bellies are full, see if mom will allow you to hold her long enough to hold the kit(s) who don't have full tummies to her so they can nurse. If the runt is warm and well-fed, then I wouldn't worry.
If you don't already have one, you absolutely need a nest box. Mother rabbits can't move kits around the way some animals can so if they wriggle out of the nest, she can't put them back and they could easily starve, freeze or even drown in a water dish. If you google "rabbit nest box," it'll bring up pictures so that you can get a good idea of the basic design. You can make your own out of wood or something like a plastic shoe-box or large tupperware/rubbermaid type container. The key aspects are that mom needs to be able to get in and out easily without the opening being low enough for kits to get out, the container needs to be big enough for mom to fit in and turn around in without being too much bigger and it needs to be cozy enough to keep the kits snuggled together for warmth (they can't regulate body temperature yet and rely on each other and the fur mom puts in the nest to keep them warm).
If you don't have a nest box yet, then once you've got one ready, you should line the bottom with hay and then gently transfer the nest that mom made into it, kits and all. Unlike some animals, rabbits will NOT reject their young because they smell like a human - it's safe (and encouraged) to gently handle the kits from the day they're born, though it appears from your pictures that you already know this. I'm guessing the box lined with a shirt is just for taking them out/handling them? If not, you don't want a shirt in the nest box - tiny claws could get snagged on the seams and if mom (or old kits) chew on the material and ingest some, it could cause an obstruction.
Around 3-4 weeks old, the kits will start to wander out of the nest box on their own - by this point, they're old enough to regulate their own body temperature. They'll start to nibble at mom's pellets, hay, etc. Not sure what type you're feeding, but if you're feeding grass hay, it wouldn't be a bad idea to mix some alfalfa in with it (introduce alfalfa very gradually if mom isn't used to it; no need to switch fully, but maybe 20-50% alfalfa and the rest grass hay once it's fully introduced). Alfalfa is higher in protein, calcium and calories, which makes it unsuitable for most adult rabbits... it's good for juveniles up to around 7 months old, though, and pregnant/nursing moms of any age. I mentioned in your other thread that she may be pregnant again - if she ends up having back-to-back litters (especially since I'm guessing she's not full grown herself), the extra nutrients in the alfalfa can be very beneficial for her.
Also, if mom's currently getting unlimited pellets, you can increase the amount you're offering her since she needs the extra calories and nutrients. In theory, she can be given "unlimited" pellets while pregnant/nursing but in practice, they probably need to be limited to some degree. Increase her pellets gradually (maybe 1-2 Tbsp more each day up to probably no more than 2-3 times what she's currently fed), making sure her poops remain normal in consistency and amount, that she maintains a healthy appetite and that she's still eating a significant amount of hay. If any of those aren't the case, then the amount of pellets should be decreased.
Mom will wean the babies herself, usually by around 6 weeks old. Normally, kits are left with mom until 8 weeks old... however, with larger litters or if some kits are significantly smaller than others, the largest kits can be pulled around 6-7 weeks to give the other(s) time to catch up. With four, you may not need to separate early at all if she doesn't have another litter, even with one being a runt. If she has a second litter, I would remove all of the kits from the first litter (except the runt if it's still much smaller) by six weeks old since the second litter will be a couple weeks old by then and nursing two litters at once is really hard on mom's body. If removed from mom, they should still be kept together and not given to new homes until 8 weeks at the youngest.
Oh, and if mom gives birth to the second litter outside of the nest box, you'll need to gently move them (and the nest, if there is one) into the nest box. I would either use a different nest box or clean out the first one shortly before the second litter arrives.
