Yeah, those burn-throughs do not look done by a 4mm spot. Do you have the units right? 4cm looks a lot more reasonable and the aperture would be a little under 11cm.
Moreover, I think you have overlooked the thermal conductivity of steel. When a spot is melted, the surrounding metal has also been heated to its melting point. Additionally, the laser lacks impact force, which means that the molten metal fluid, as long as it hasn't flowed away, will continue to absorb heat, raising its temperature above the melting point and further melting the surrounding metal.