So you know that this triangle must be a 45-45-90 degree triangle (you can tell this from the picture!)
The side that is length 4 is opposite from the 45 degree angle, and remember that congruent angles also give you congruent sides. This means that the length of a must be equal to the length of 4, because both are located opposite a 45 degree angle.
To find b, you must know that the hypotenuse of a 45-45-90 triangle is just equal to a[tex] \sqrt{2} [/tex]. Thus, the length of b= 4 rt. 2