True, she could only coax the Time Turner into giving her four hours at a time. But once those four hours were granted, who's to say she couldn't request more? If she took it in skips, it was possible to go back further. Or forward further, though she felt uneasy trespassing on the future. She became intimately acquainted with time-sickness, though she supposed that was the price she must pay for scholarly advancement. There were always costs for those sorts of things.