''Anyone can sit down and write some boring artistic song,'' says Britney Spears. ''Pop music is the hardest s--- to write.'' Which is what Britney's hard at work on for her as-yet-untitled fourth album, due this fall. ''The subjects are really out there. It's about general things that teenagers go through that are real but nobody talks about.'' (Unlike her new hair color, which is the source of much debate.) ''I really don't think that many people from my core audience will get it,'' she says of her new material -- including one song in which she raps. ''But I would rather somebody either really love it or really hate it. I don't want anything in between.''