That's NOT right. 4mm machined off the mounting surface of a wheel at ET40 will INCREASE the offset to ET44. A +40 offset means the mounting surface sits 40mm away from the centre of the wheel, towards the outer rim. Machining 4mm of material off the mounting surface will increase the distance of the mounting surface of the wheel to the centre by 4mm, hence changing it to a +44. Or just think of it this way - if adding spacers ("adding material") lowers offset, then machining off material should mean an increase in offset.
dylan8 also has a point about offset being dependent on the width, but we're assuming here that all the wheels (ET40 and ET44 variants) are of the same width?
Edit: dylan8 beat me to it! The paint fumes are doing you good mate.
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