已知正数 x,yx,yx,y 满足 9x2−1+9y2−1=9xy\sqrt{9{x}^{2} - 1} + \sqrt{9{y}^{2} - 1} = {9xy}9x2−1+9y2−1=9xy ,则 4x2+y24{x}^{2} + {y}^{2}4x2+y2 的最小值为___.