
Your pups are bound to weigh less than a smaller litter would, I expect their birthweights were less and it will take them a while to catch up. But in my experience they do catch up and eventually reach the size they are genetically programmed to reach.
I think at 4 weeks your bitch will be finding feeding a bit uncomfortable - all those teeth! Also she will need to regain some condition herself after carrying and feeding her big litter so I would persevere with the weaning for her sake as well as the pups.
As for food: I find Burns minbites soak well and quickly too. But what they really like is raw minced meat - beef, tripe or lamb. I usually start by hand feeding the pups on my lap with little balls of meat on their tongues, one at a time. With a big litter this takes a while! Once they've got the hang of it you can mix in some of your complete food and then move on to putting the food down for them to tuck into. By about 6 weeks I give them a bowl each to help them get ready for their new homes. You can also try putting down raw chicken wings for them to chew on. They are certainly not too young and you'd be surprised how much they enjoy them. They won't make much inroads at first so you can then mince them up afterwards and add the minced chicken wings to their next feed.
I'm not sure about quantities. I usually give them what they will eat, if it all goes quickly I give them some more.
BTW your bitch is a good girl to lie down when you make her :-)