How do I get a pay raise?

So I just found out a junior member of my department makes a lot more money than I do. I have been with the compnay almost 4 years and I am the only one qualified to do "his job" and I have been doing it for about 2 years now. "He" joined the department almost a year ago and though technically senior to me in billet, still has yet to take over any of the resposibilities he was hired for. Meanwhile, I am single handedly doing 90% of the technical work. Should I just quit? I went to my boss about 6 months ago and told him I expected to get the title and pay for the job I was doing but the issue has since gone dead.