Canon MG4120 - Reset Ink Levels on the Cartridge
Unfortunately, the cartridge ink levels cannot be reset on the Canon MG4120. Every time a page is printed, or a head clean run, the printer writes this information to the copper contacts on the cartridges. The cartridges cannot be reset.
You can however bypass the "ink empty" message by holding down the "stop / reset" button on the printer for around five seconds for each cartridge. The printer will allow you to continue printing.
Youtube Video: Override the empty cartridge message.