The K4900 manual has a convergence adjustment procedure, and can be found on arcarc.xmission.com in the monitor section.
But here's a quick recap:
There are 6 rings on the neck, in groups of 2. First you would loosen the clamp screw, then use a razor blade to cut through any glue so that the rings can move freely. Trying to force a stuck ring can make the ring crack and break off, thereby making your convergence even worse.
As for the rings, you'll see the 3 sets of two with a plastic spacer between each set.
1) The set closest to the neck board are for adjusting your red and blue images together. It's best to get into a test screen or something that stays the same and has some white, so you can adjust them until you get as close to a perfect purple as possible. You may sometimes have a tiny hint of red or blue showing that won't go away completely, so you'll have to find the best spot. Sometimes moving all 6 rings closer or farther away from the tube on the neck will help with the last bit of convergence.
2) Once you have your red and blue aligned together, the middle set of rings will help line them up with the green. Once the purple lines up with the green, it will make white.
3. The third set (closest to the front of the tube) is for centering the picture. If you unplug the yoke connector, you'll get a dot in the center of the screen. (make sure you turn the brightness down until it's barely visible). You can then adjust the third set of rings to get the dot as close to the center of the screen as possible. That will ensure the image is centered as best as possible when the yoke connector is reconnected.
After you have made your adjustments, carefully retighten the clamp screw (not too tight or you'll break the neck) until the entire ring assemply no longer spins. If your rings are still loose, you can put some hot glue on them to hold them in place if you think they move on their own during play....