My 4900 is almost a year and a half old, and since I installed the humidifier I've had no problems with it whatsoever. I print much more B&W than colour, so half of the ink cartridges are the originals that came with the printer. They are still several months away from their expiry dates.
I've read elsewhere that they should be taken out and shaken every six months or so, but I hesitate to do this, fearing that air would get in the tubes.
Am I being overly cautious? Should the cartridges be shaken from time to time? I can't say I've noticed any shift in the printed colours.