we know that if a right triangle is isosceles, then the 2 sides that are equal must the legs. this is because the hypotenuse must be the largest side: if there is another side as long as the hypotenuse, then there will be 2 right angles, and that can't happen!!!
knowing this, we can set both legs to be x and use the pythagorean theorem
x^2 + x^2 = (4(2)^1/2)^2
x = 4
let me know if you have any questions!!!!