Question 1034868
If you replace x with 4 in the given function the answer becomes root(-1).

The root of negative numbers does not exist in the real number system.

This is why x cannot be 4. The value of x = 4 stands outside the domain of the 

function. 

Prove:

f(x) = root(x - 5)

f(4) = root(4 - 5)

f(4) = root(-1)

So, x = 4 is undefined for this function.