x^2 -4x

First of all, let's solve the expression like an equation. For this reason, we'll put:

x^2 -4x = 0

We'll factorize and we'll get:

x(x-4) = 0

We'll put each factor equal to zero.

x = 0

x-4 = 0

x = 4

Now, we'll follow the rule: between the solution of the equation, the expression will have the opposite sign of the coefficient of x^2, that means that x(x-4)<0, outside the solutions, the expression will be positive.

So, the conclusion is that the expression is negative for values of x which are in the interval (0,4).