You may have entrained air from the nozzle orifices, and may not have a clog at all at this point. Drop outs may be a property of air, rather than of clogs, in other words. It is commonly suggested that you make prints that exercise your ink channels, and rest the printer, between cleanings rather than perform serial cleanings. Uninterrupted serial cleanings produce, in my experience and opinion, air entrainment. Room humidity probably plays little role in the air entrainment problem.
It the printer were mine, I would perform no further cleanings and would make a print that strongly exercises the Cyan channel. As has recently been discussed here, the values to limit your ink usage to Cyan alone are not readily obvious, but if you build a PS file using hex value 00ffff, as has been presented here:
http://www.marruttusa.com/printer-maintenance/inkjet-printer-purge-files.php#col , and print that without color management you'll be pretty close to a pure C usage. If this has no impact, let the printer sit overnight after a formal shutdown, and start over with printing (not with cleaning) tomorrow.
That you've used your 4900 for 6 months, without printing a nozzle check until today, is impressive. You are having better results than I have had, for sure.
Others will be able to chime in and address gaps in my advice I'm sure. Mark Segal and Wayne Knox, among others, are quite knowledgable in this area.
John Caldwell
