That's my 4-year-old son's most frequent question these last few weeks. He's obsessed with letters and spelling (and now with writing too). So lately he's been asking me to spell the darnest things for him, like "Whoa", "Ummmm", and "Dying" (he doesn't know the meaning of that word, but he heard it on a Wiggles' song that he loves, "Gypsy Rover"). I didn't want to teach him that one, for fear that he'd repeat that aloud in his preschool in front of teachers and other children.
By the way, if you have a kid at home with any degree of hyperlexia, or who is starting to learn how to read (and loving it) you have to go to this website: Starfall. All teachers swear by it, and we surely agree it's wonderful. In my case, it's a way to help my son pay more attention to a narrative, since the stories are animated and interactive. Besides, he'll be able to realize that each letters' phonic sounds combined make a word, so he doesn't need to rely on visual memorization alone, he'll be able to read novel words.