Like i said two posts above yours, it was 4kg from his Vuelta weight, not his Giro weight.
I think he could realistically lose 2kg before the Giro, and as Didinho said, it's possible that they plan on losing a bit more by the last week of the Giro. (Though i'm not convinced losing weight during a GT is something to strive for, Dumoulin actually gained a bit of weight during the Giro he won).
It is also possible the 4kg claim was made either a while ago, or was a situation of a while ago which Bruyneel only heard about recently. Maybe at this moment, it's only 3kg. Maybe the claim was exaggerated and 3.1 kg became "more than three" which became "between 3 and 4" which became "almost 4" which became 4. Who knows.