4 Vertex Cover Problem is not NP Complete why?
If your graph has $n$ vertices, there are $O(n^4)$ possible subsets of $4$ vertices. Given one such subset, it is easy to check if it is a cover in $O(n^2)$ operations, by inspecting every edge and verifying whether it has an endpoint in the given subset.
Thus, in $O(n^6)$ operations we can check all possibilities.