It's hard to know for sure at this point, but I will try to help you trouble shoot your problem and get back to happily printing. Unfortunately, problems like what you're experiencing are very common with this model, though we have had quite a few reports of success from people in your situation who have fixed their printers with our help, so there's hope
Generally, when the exact same nozzles are missing after doing cleaning cycles and manual cleaning, this usually indicates a blockage in the print head. If the missing nozzles move around after cleaning, then this is more likely to be restricted flow caused by build up in the damper(s). If the exact same nozzles are missing in the black channel after doing a black ink change, then I suspect you may be dealing with a blockage in the print head- as the two blacks have different damper channels, but share the same print head channel. The wiper blade also plays a big roll in the cleanliness and function of the print head. The 4900 is a very different beast compared to the previous models that I have a lot more experience with, and has MANY more sensors and mechanical parts that increase the factors when trouble shooting problems.
What is the humidity in your printing environment? If your climate is too dry, it will certainly cause clogging and flow issues. We follow the industry standard, and maintain humidity levels between about 40-60%. I find 35-50% is excellent for the best results with your printer, inks, and papers.
I Hope this helps. Please let me know if you have questions or there's anything else I can help you with.
Best regards~ Dana