4-6 weeks is way, way too early. The shelter for which I volunteer wants them 12 weeks at the youngest (but they're able to be fixed slightly before that, depending on size). I took in foster kittens last year that were somewhere around 4 weeks. They were very, very small, but they were able to eat wet food (it seemed like they'd been on their own and had been hunting bugs and whatever they could eat- they were found under a trailer, mom had abandoned them).
I still have them, my older girl cat adopted them and became their mama

:. One cat is pretty normal, the other is very sensitive and needy, and he still has the need to nurse on everything in sight, poor baby. That's a big problem when they're separated from the mom so young.