For four years, he was just another member of the Legion of Boom. Walter Thurmond III was destined to be a mere Richard Sherman prop, toiling in relative anonymity behind the brashest cornerback in the NFL. But this is the year that all changes. On another coast, in another division, playing for a Giants secondary that doesn’t have a nickname (yet), the fifth-year corner believes he is ready to break out. By the end of 2014, if Thurmond has his way, you will know him as the finest slot cornerback in the NFL. “Most definitely,” Thurmond told The News on Thursday. “I think so. I’m looking forward to having a big year and contributing in any way I can.” In Seattle, Thurmond was just another corner, one of the many sidekicks working with Sherman and safety and interception machine Earl Thomas. But here, he will be relied upon, a key piece of the Giants’ revamped secondary. The 5-11, 190-pound Thurmond thrived in the slot last season in Seattle, routinely containing inside receivers, and he was signed to handle just that for the Giants. It’s a role that has become critical in the pass-happy NFL, and subtle rule adjustments stand to make slot corners even more important. Officials are spending the preseason restricting the contact defenders can make with receivers, so speedy, agile corners who can cover without jostling are in higher demand.