Most puppies can begin eating solid puppy food by 4 weeks of age.
You should start out feeding the 4 week old puppy some wet solid puppy food so that it's easiest for them to eat and then as they get there teeth in more then you can switch them to dry puppy food.
Puppies will usually begin weaning off the mothers milk at 4 weeks of age and will be usually fully weaned from their mothers milk by 6 weeks of age.
You can start introducing the puppies to the solid puppy food at 4 weeks of age but some puppies may not be ready to eat the puppy food that is solid until around 6 weeks of age.
They will naturally begin eating the solid food as there mother weans them off the milk supply and they will also begin drinking water on their own as well.