Disassembled another 4900 cart. There is just one connector to let the ink out. No pressure chamber, no extra air bag around the ink pouch. The ink bag flattens when ink is taken out and the near empty one I have is quite flat so no pressure is build in the ink bag itself either.
It got the ink flow sensor that connects with two steel springs to the cart chip that connects to the printer electronics like the 11880 got first and the 9900 later. Might give the same error with a new cart when there is a small air bubble on the sensor. Of course the 11880 and 9900 etc have an air pressure chamber.
If chip resetters exist for the 4900 carts I would refill them from a tip on the bag itself. The sensor in the ink connector is making things worse to get a backflow valve out, ink path in the connector no longer straight either. Drilling a small hole at an angle along the sensor and filling the hole at the outside might be possible but will not be easy.
If pressurised I think the 4900 has an ink pump internally.
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Met vriendelijke groet, Ernst
http://www.pigment-print.com/spectralplots/spectrumviz_1.htm
April 2014, 600+ inkjet media white spectral plots.