If $(1+x+x^2+x^3)^5= \sum \limits_{k=0} ^{15} a_k x^k$ then $\sum \limits_{k=0}^7 a_{2k}=$

Question

$$\begin {array} {1 1} (1)\;128 & \quad (2)\;256 \\ (3)\;512 & \quad (4)\;1024 \end {array}$$