How old is the youngest cat to ever have kittens?

Sorry but I don't buy your story - that means the kitten was only 8 weeks old and got pregnant. A tom cat would have killed her in the mating process. And the earliest a kitten comes in heat is 4 months. So it would not be possible for your kitten to have kittens at 4 months old.

Besides that she could not birth them safely - she would be too small.

You are incredibly irresponsible to let that happen. She shouldn't even have come into heat that young. Her body is too small. Her kittens may appear to be doing well right now at only 3 days old, but you can expect a whole mess of genetic "issues" on the road ahead. I would put money on most staying undersized, and at least have having organ failure before they hit 3 months old.

PLEASE keep your intact pets away from other intact pets, and get that poor kitten spayed. Her body isn't even half way done growing. Expect some major bone issues later in her life (as early as 6 years) as her body that should be building it's own calcium reserves is now producing all of it in milk.

Here is a pic of the cuties.