Generate all possible quadruplet and increment counter when sum of a quadruplet is equal to n
for ( int i = 1 ; i < n ; i ++ ) for ( int j = i ; j < n ; j ++ ) for ( int k = j ; k < n ; k ++ ) for ( int l = k ; l < n ; l ++ ) if ( i + j + k + l == n ) counter ++ ; return counter ; }