If x < 4, then x - 4 is negative, and we cannot take the square root of a negative number. Therefore, the expression is undefined for x < 4.
If x = 4, then the expression becomes:
sqrt(4^2 - 8(4) + 16) = sqrt(16 - 32 + 16) = sqrt(0) = 0
If x > 4, then we can simplify the expression as follows:
sqrt(x^2 - 8x + 16) = sqrt((x - 4)^2) = |x - 4|
Therefore, the simplified expression is |x - 4| for x > 4, and it is undefined for x < 4.