I found this Instruction:
“Remove all cartridges and turn the printer off for at least five minutes. The printer will indicate “NO cartridges” after turning back on, reinstall all carts so they lock snugly into place and close the cartridge bay doors.”
One further thought, along the lines of resetting pram and such on the computer, actually disconnect the power cord for several minutes after pulling carts and turning power off. Then plug back in and continue as quoted above. I don’t have a 4900 and have no idea if it will make any difference.