A dual NAND install on a 4GB Corona console *IS* possible, but it is far from easy to achieve.
You need to remove the 4GB eMMC/BGA NAND and replace it with a 16MB NAND chip.
After doing that, you need to change the resistor configuration underneath the board so that the Southbridge can talk to the new NAND.
My advice to you is to swap your console for a 16MB model of pay someone with the skills to do the conversion.