The gradient of the tangent line to the function at x=4 is -1/10.
The gradient is negative, so the function is decreasing at 4.
The words slight and steep are judgemental. But for sure, -1/10 is not steep. It is close to zero, and therefore close to horizontal.
Remember, the value of the derivative at a certain point is the same as the gradient of the tangent to the function at that point. It is important to let that statement roll around in your head for a while. Looking at graphical illustrations helps.
