While I now use a 3880, I still have my 4000 sitting around.
I spoke with a knowledgeable Epson technician and described a similar situation with my 4000. Quite often I'll do the ink test, and maybe a small portion of one ink channel has some missing areas. So I do a cleaning, and I completely lose three more ink channels.
Explaining this to the technician, he suggested that perhaps air is leaking into the ink lines, and that would explain this counter intuitive behavior of the machine. (That is, that it gets worse after a cleaning. How neat that would be?!) He also suggested downloading the Epson 4000 repair manual and checking the symptoms of my printer against those in the book. I forget the exact name of this manual, so that it could be googled.
He also said that he has a 4000 that doesn't get much use, and he rarely has a problem with clogging. When I first got my 4000, I rarely had a clog, even after a couple of weeks of not being used. It was only after it had aged that it began having a clogging problem. This would also be consistent with the possibility that air is leaking into the lines.
So before selling it, I'm hanging onto the the 4000 until I have a chance to take it into a repair place and test this possibility. I'd rather sell a properly functioning printer.
Weren't the 4000 and 7600 based on the same technology?