0.4 ohm coils are low resistance, sub ohm vape coils that do not require a lot of power to heat up. They are typically found in DTL (direct to lung) devices, but can also be found in many MTL (mouth to lung) devices too. As more power can be used with these coils, they can be great for producing thicker vapour clouds for vape tricks and provide lots of flavour from e-liquids and nic salts.