Seriously. The only way you can guarantee that a 4-year-old with a tennis racquet will go to the US Open is to buy them a ticket. The single most important thing is that the child wants it. If they do, you'll know. From there you can seek reliable advice about what path to follow. But it scares me, really really scares me, to hear people ask questions like, "I'm going to give a 4-year-old a tennis racquet/baseball glove/ice skates/basketball for Christmas, how do I turn them into a successful professional athlete?"