Probability that four geometric variables sum to six

Let $X_1,X_2,X_3$ and $X_4$ be i.i.d. discrete random variables with the probability mass function

$$P(X_1=n)=\begin{cases}\dfrac{3^{n-1}}{4^n}, & n=1,2,…,\ 0, & \text{otherwise.}\end{cases}$$

Then $P(X_1+X_2+X_3+X_4=6)$ equals ____________ (round off to 2 decimal places)