If it's an indexed shifter, meaning you click it and the derailleur moves, then try adjusting the derailleur first. You may need to tighten the cable, though, as sometimes too much slack will cause improper shifts. If you haven't had it tuned up in 4 years and are riding it quite often, it's probably time for a basic tune-up at your LBS. It's not mustachian and I've never done it, but paying $40 for someone to go through everything and take care of issues that may not have arisen yet, may make sense for some.