At four weeks they should be fine to start on kitten food. I would get some wet food and some dry kibble to try. You may need to soak the dry at first with some water. The wet food you can just mash up and push a little to their mouths with your finger to get them to lick it off. If you're not having any luck with that. You can mix some wet with some formula milk and suck some into a syringe get them to lap it from the syringe and don't force it into them. Eventually they will get the taste of it and try what's in the plate/bowl.