Adding brute force, better and optimal approach solution for 4Sum problem using C++

[s-bhalode]

Hey @akkupy I found that currently in the C++ folder, the solution of 4Sum problem is not that much understandable, So I would like to explain that via brute force, better and optimal approach solution. Please assign me this issue, I would like to work on it.

[s-bhalode]

@akkupy Please add the respective hacktoberfest labels also