My cat is still nursing her kittens that are 8 months old . She recently became pregnant again and delivered 4 kittens that were stillborn.
Could that be a cause of her still nursing the kittens from her previous litter?
I am not a cat expert, but with rabbits it would be highly likely to see the kits be stillborn when the mother was still nursing. There is some related information about rabbits here
Additionally it looks like your 8 month old kittens could be sexual mature, you do not mention having any of the kittens fixed, so it is possible that one of the male kittens was the father of the stillborn kittens. Inbreeding can be another cause of stillbirth.
The only definitive way to get an answer about why they kittens were stillborn, is by having a veterinarian look at the stillborn kittens.
This is a tough question and something I had actually researched often due to a similar issue. Some queens, like 10% or so, lack vital nutrients and have trouble with birth altogether. My queen Phobos I rescued as an abandoned baby (hypothermic, lethal and wasn't supposed to live) 3 years later she is happy, healthy, but a bit evil lol. She was pregnant 3 times: the first she had 4 stillborns, the second she had 2 and 2 but only one survived the first days, last litter she had 1 stillborn and lost her other 3 in 2 days. I had vets highly involved and we'll kept a nutrition diet and she just isn't meant to have babies, sadly. All the blood work showed nothing wrong.
The veterinarian thinks that she was a malnourished runt and her small badly just isn't capable to carry to full term. The reason her only baby survived is because the others in the litter didn't live inside long enough and made extra room.
If the kittens weren't born alive I don't think the older babies potentially could've taken the nutrients the mother needed, also if the kittens were born healthy the first few days to a week of milk the babies need. That milk cycle is healthy for the baby from the cat eating after birth, etc. It helps the newborns build immunity and has a lot of beneficial effects for them.