''The Fourth Horseman'' opens with the arrest of Laurie McAllister, her brother Alex and her friend Javed. Her father, a research scientist, is being carted off to hospital with a head injury, the lab is on fire and its squirrels have been released. The police, naturally, are taking a dim view of proceedings. You might think that a simple story of earnest teenagers fighting the good fight against vivisection will ensue, but you will be wrong. This is a considerably more complicated book and we zip straight back to how it all began.