My code

… was written in C++ and can be compiled with G++, Clang++, Visual C++. You can download it, as well as the input data, too. Or just jump to my GitHub repository.

       
#include <iostream>
       
 
       
int main()
       
{
       
  // always a 20x20 matrix
       
  const unsigned int Size = 20;
       
  unsigned int matrix[Size][Size];
       
 
       
  // read from console
       
  for (unsigned int y = 0; y < Size; y++)
       
    for (unsigned int x = 0; x < Size; x++)
       
      std::cin >> matrix[x][y];
       
 
       
  unsigned int best = 0;
       
  // walk through all cells of the matrix
       
  for (unsigned int y = 0; y < Size; y++)
       
    for (unsigned int x = 0; x < Size; x++)
       
    {
       
      // three more horizontal cells (right)
       
      if (x + 3 < Size)
       
      {
       
        unsigned int current = matrix[x][y] * matrix[x+1][y] * matrix[x+2][y] * matrix[x+3][y];
       
        if (best < current)
       
          best = current;
       
      }
       
      // three more vertical cells available (down)
       
      if (y + 3 < Size)
       
      {
       
        unsigned int current = matrix[x][y] * matrix[x][y+1] * matrix[x][y+2] * matrix[x][y+3];
       
        if (best < current)
       
          best = current;
       
      }
       
      // three more diagonal cells (right-down)
       
      if (x + 3 < Size && y + 3 < Size)
       
      {
       
        unsigned int current = matrix[x][y] * matrix[x+1][y+1] * matrix[x+2][y+2] * matrix[x+3][y+3];
       
        if (best < current)
       
          best = current;
       
      }
       
      // three more diagonal cells (left-down)
       
      if (x + 3 < Size && y >= 3)
       
      {
       
        unsigned int current = matrix[x][y] * matrix[x+1][y-1] * matrix[x+2][y-2] * matrix[x+3][y-3];
       
        if (best < current)
       
          best = current;
       
      }
       
    }
       
 
       
  std::cout << best << std::endl;
       
  return 0;
       
}

This solution contains 4 empty lines, 7 comments and 1 preprocessor command.