Let f(x)=∑n=0∞anxnf(x) = \sum_{n=0}^{\infty} a_n x^nf(x)=∑n=0∞anxn. If f(x)=xexf(x) = x e^xf(x)=xex, what is a4a_4a4?
14\frac{1}{4}41
16\frac{1}{6}61
124\frac{1}{24}241
111