I was given $\displaystyle \int^4_0 f(x)\,dx = 4$

I was asked to evaluate the following:

$\displaystyle \int^4_0 (3*f(x)+1)\,dx$

What I simply did was took $\displaystyle 3*4+1=13$ and I was marked down a point. How is this incorrect? I'm sure it's something stupid I am not seeing but I can't seem figure it out!

Thanks