If a child learns the trick of reading before it goes to school, well and good, but there is no need to push it. Just enjoy looking at books together and reading stories. If a child looks at the book with you (both the writing and the pictures) as you read the story, it will learn an awful lot about how reading works. Watch a child's eyes when it is is looking at a book with someone who is telling the story – it's fascinating .
If your grandchild is already reading some words at four, humbert, he will have no trouble carrying on. As others have said, any suitable children's books will be fine; they don't have to be of one particular kind. Well, maybe they do, actually — one he enjoys rather than ones he doesn't enjoy.
Just by the way, for anyone who's interested, I hate the notion of stretching children as if they were elastic. I know what it's supposed to mean but the expression leaves me with an uncomfortable feeling.