The side length of an isosceles right triangle is 1/√2 times the hypotenuse length. If x is the side length, then ...
x = 4/√2 = 4(√2)/2
x = 2√2 ≈ 2.83
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You have not specified the form of the answer. Sometimes an exact answer (2√2) is required. Sometimes the answer is required to be rounded to tenths or hundredths.