Each part is important, but it's important that people who will do the best job on that part sit in that chair. For whatever reason your leader didn't think you were the best person for that lead book. It shouldn't matter to you -- your job (actually the job of any/all members of an ensemble) is to do what it takes to help the ensemble sound its best, and if that means playing the 4th book, then do it as if it were the most important book in the band. Don't ever look on your assignment as "punishment" or "demotion" but rather look on it as a re-apportioning of the band's resources to get the best result. When I used to conduct a community orchestra, I was floored at the first rehearsal at how much the string players' egos were tied up with which stand they were at and whether they were inside-chair or outside-chair. I'd never run into such egotism in any of the school bands or community bands I'd played in or conducted. Don't let your ego get in the way of being a great team player.