Answer: 5.657
Explanation:
Split it into two right triangles, with the hypotenuse being the diagonal.
Then, a square plus b square = c square
So, 4 square + 4 square = c square
16 + 16 = c square
32 = c square
square root of 32 = c
square root of 32 = ~5.657
c = ~5.657