Just take the key out of the ignition. The reason your battery went flat was because the immobiliser was reading the chip in your key for 4 days straight, which drained it. The way the immobiliser reads it is through some sort of radio wave by pulsing to the chip inside the key and the chip pulses back in return based on the power sent by that initial pulse from the immobiliser. The chip itself has no power source, so that's why the battery went flat, because the initial pulse will take some measurable amount of power. Take the key out. If you leave the key in and your car is stolen, comprehensive insurance WILL NOT cover your car for theft because you left the key in the ignition.