A .4ohm build will pull almost 20 amps from your batteries. Just letting you know. Be safe with your builds, use only batteries rated for your builds, etc. Is there a reason you're wanting only builds in that range? With a series box you cam do better at higher ohms. If you go that low, your build is somewhere around 150 watts. A 2 core, 28 gauge claptoned with 40 gauge, wrapped around a 2.5mm 8 times will get you close to .4 ohms. Vape might get a bit hot.