Every few days, whether I print or not, my 4000 goes through a major cleaning cycle. After that at least one of my colours disappears. It usually takes a couple of minor cleaning cycles to get it back, sometimes more than others.
From my research it seems that bad sealing between the dampers and the head, especially on early model 4000s lead to this. Newer damper design is supposed to fix the problem.
I am just trying to get it last until a printer comes that I feel comfortable getting as a replacement.