Beautiful!
Since you say you are new to rabbits and getting 4 week old rabbits is a big challenge even for an experienced rabbit person, I will tell you how normally things should go with the babies.
I understand that these babies were abandoned because their mother had another litter, this wasn't your fault just now you have babies and they should still have their mother's milk for another 4 weeks. Which is impossible right now so we have to go with what we have at hand.
Also if they were fed carrots and apples that was surely wrong, you are generally doing good that you got junior pellets for them, and they are already based on alfalfa so you can feed a bit more alfalfa since they are underweight but not necessary, I would suggest feeding unlimited timothy or other grass hay (alfalfa is not grass). Also you can offer them 1 tbs of kitchen rolled oats once or twice a day, they are good for underweight and weak rabbits, but when they will gain some weight you can reduce that.
Generally their diet should be unlimited timothy (or other grass) hay and unlimited junior pellets min 16% protein in them, plus 1-2 tablespoon of rolled oats. Plus unlimited fresh water, I would recommend boiled since their stomachs still aren't strong, just to stay on safe side.
What would be their normal life and diet if they were staying with their mother.
1. They get mothers milk for first 8-10 weeks.
2. They get mother's poo's, they need to eat them to get right bacteria helping them to digest adult food
3. They starting eating hay from very close to their birth, at about 2-3 weeks they eat little more hay and by week 4 they should eat hay and they should love it. They start drinking water then, since milk doesn't provide enough liquide anymore.
4. Around week 3-4 they start trying their mother's pellets as well, by week 6 they eat up to 50 gram of pellets, later they eat more and more pellets and it is important that they have unlimited pellets, they can eat up to 200 gram a day, that is 4 adult daily portions. They need much protein, calcium for their bones too, so you can offer them 50 gram of pellets 4 times a day (don't give too much at a time, better less but a few times a day).
5. Water should be unlimited and I would suggest boiled.
So after 6 weeks babies are usually eating full portion of solid food and still getting mother's milk, around week 7-8 she stops nursing, but some mothers are so nice they keep feeding later, my Arnie fed up to 4,5 months, I felt bad for her, she is an amazing mother she fed 4 times a day when normally rabbits only nurse once or twice a day. Her kits are exceptionally strong, healthy and well behaved, they are very good to each other as well. But that would be an exception, normally 7-8 weeks would be enough.
So as I said in your case I would offer unlimited timothy or grass hay (add limited alfalfa if you want since they are underweight), unlimited water, unlimited (up to 200 gram a day, 4 times x 50 gram for both, if they look hungry you can increase that), plus 1 tbs of rolled oats twice a day.
No vegs or greens until they are 4 months old, but you can offer some dried herbs, mint, chamomile etc.
That would be their diet for the next couple months, when they are 4-4,5 months you should start limiting their pellets and switch them gradually to adult pellets, then you can start adding greens/vegetables, but just one type at a time, we can discuss later.
