depends on the draft. my first estimate was that it would have an draft of 8 meters, which, with an Cb of 0,5 (which is on the low side) the displacement would be 8*15*100*0,5= 6000 tons. if we would go for 5 meters, 5*15*100*0,5= 3750. the Cb would most likely be between 0,5 and 0,65 for an vessel like this, so 4000+ at least would not seem that weird to me?
that where that type of math is going to go wrong:
if I do that to the Fridtjof Nansen class I get 8554 tons when see weight just 5200 tons (7,6*16,8*134*0,5) but if I give it a CB of 0,30395...
In some fashion that type of math you do is calculating the weight of an box, and as we all know a box is not an hull! At the moment I don't remember but my teacher back in the days when I did go on an engineering school he learned us to do four three math on the hull, do the same calculation as you do but we calculated the hull at three area: "Aft, center, bow" and used them to get a more accurate number!