Is the answer 2.515 ?

Suppose that the feight of water in the container =hcm and the side of the cube is x cm .

Now from conservation of mass

pi 3^2 h + 0.9x^3 -x^2h = 4^3.............(1)

And from archemedes' law

x^3 *0.9 = x^2h ......................(2)

from (2) h=0.9x

from (1) h= 64/9pi

x = 64/(9pi*0.9)= 2.515 cm