I have F(x)= x^4 + 4x^3

I need to find out the vertical asymptotes with

lim x-> a+ f(x) = + - infinity and

lim x-> a- f(x) = + - infinity

With a rational function I set the denominator zero and the x-values I get I use as "a"

How does this work for a polynomial?

If I set f(x) = 0 I get for x= 0 and x= 4

I can not figure out what to do next...

Any help?

Thanks