You should certainly ask some combination of your boss, the HR department, and other managers you know, how promotions are determined.
It really depends on the organization. In some places the correct thing to do with an expanded role would be (with your boss's co-operation) to create an updated job description and take this to HR (or whoever else has authority) to be re-graded.
If the new grade is higher than the grade for your original JD from 4 years ago, bang, you're promoted. If it's not, then you've discovered that your additional responsibilities don't (yet) add up to the responsibilities of someone at a higher grade than you.
Naturally not every organization has a process this formal. But perhaps the company does have common standards for what constitutes the higher grade, in which case you need to find them and demonstrate that you've achieved them. Alternatively you might find them and discover that you haven't achieved all of them, in which case you need to work with your boss to modify your role to cover the ones you've missed.
Some organizations (especially small ones) might not have much formal definition of different grades at all, in which case it's pretty much between you and your boss to plan a route to promotion.
Finally, you could sidestep the whole business by applying internally for a post with a higher grade. Your organization doesn't necessarily offer a smooth career path with your role gradually passing through the grades. Rather, you might be expected to take a series of one- or two-grade steps through different roles. Again though, it's useful to involve your boss since he can help prepare you for different roles.
It's possible that your boss doesn't want you to be promoted, in which case you have a problem. If he won't answer the question "what should I do to get promoted", or won't help you negotiate the process, then for whatever reason he wants you stuck where you are. There might be someone else in the organization you can go to for help. But ultimately if you think you're ready for promotion and the organization thinks you're not, then the only way to really prove them wrong is to get a more senior job elsewhere. Assuming that you otherwise like your current employer, the trick is to establish what your options are short of leaving. Once you've exhausted them, you may discover that you're simply more ambitious than your employer allows for.