When can kittens eat solid/canned food?

I have been raising 3 baby kittens from the day, yes, the day they were born. Their mother was young and ran away shortly after they were born and other cats started attacking them. Well long story short, I rescued them and now they are playful little 4 week old kittens. Their eyes are open wide, they walk, well run, without difficulty, their ears are perked up, they are attempting to jump and pounce on things, and are very vocal and playful. I heard you can't wean them until 8 weeks, but I also heard you need to introduce them to soft canned food at 4/5 weeks. Please help me, our other cat eats canned food on a daily basis so it wouldn't be hard for us to get it.