Get the first four minimum values in a matrix
<p>I have a matrix:</p>

<pre><code>X =
     0    81    13    15   100     2
    11     0     6    10   200     8
    19    22     0    20   300    23
</code></pre>

<p>I want to get the first four minimal values in the whole array <code>X</code> with the indices of each value in the array. For example I should get vector <code>v = [2 6 8 10]</code> and the index of each value in <code>X</code>. 
Also, I want to ignore the zero values when the row number equals the column number.  </p>

<p>I have tried to use the <code>min</code> and <code>sort</code> functions, but I am not sure how to do it. </p>