Rear (what I assume is a 7 speed) derailleur only covering 4 gears on my rear cassette. I inherited my bike last year, so I don't actually know what the parts are. My rear derailleur is labeled Shimano 7 SIS, so I assume it's a 7 speed. Makes sense, there are 7 gears on my cassette. However, my derailleur only covers 4 gears. On first speed, my chain is properly sitting on the first gear, but at the seventh speed, it's barely on my fourth. I've followed the instructions on Sheldon Brown's page, and only by tightening my High-gear limiter was I able to get even the fourth gear covered. I've played around with the index adjustment, but all it appears to do is tighten the shifter without changing anything about the derailleur. 

So my question is... what is there to do, aside from take it to my LBS? 