This is similar to this question about metric measurements. In Australia, a common measurement is 90mm x 45mm for timber. A 4x2 would be (approx) 100mm x 50mm but, as I understand it, that is the milled rather than final measurement. Does the smaller metric size refer to the final measurement or does have nothing to do with a 4x2?
It's a bit wider than a typical US 2x4, but likely equivalent. A 2x4 that you get for building is 3.5"x1.5" so that would equate to 89x38mm.