Two dimensial array. Multiple selection dropdown

I have a php form that has a known number of columns (4) and unknown number of rows. In that form I use single selection dropdown, free input text, radio and multiple selection dropdown.

            <table>
            <tr>
                <td><b>Single selection dropdown</b></td>  
                <td><select  name="row[0][column_1]" >
                    <option></option>
                    <option>Selection 1</option>
                    <option>Selection 2</option>
                    <br />
                </select> </td>

                <td><b>Free text</b> 
                    <input type="text" name="row[0][column_2]" placeholder="Free text">  </td>   
                <td><b>Radio</b> 
                    <input type="radio" value="ja" name="row[0][column_3]">Ja
                    <input type="radio" value="nein" name="row[0][column_3]">Nein
                </td>   

                <td>
                    <select multiple="multiple"  name="row[0][column_4[]]">
                        <option value="k80">K80</option>
                        <option value="k50">K50</option>
                        <option value="hitch">Hitch</option>
                        <option value="zugpendel">Zugpendel</option>
                        <option value="Piton-Fix">Piton-Fix</option>

                    </select>
                </td>
            </tr>
        </table>

At the moment everything work as expected but I cannot get data from multiple selection dropdown.

$row = $_POST['row'];
 echo '<table>';
 foreach ( $_POST['row'] as $val)
 {
    echo '<tr>';
    echo '  <td>', $val['column_1'], '</td>';
    echo '  <td>', $val['column_2'], '</td>';
    echo '  <td>', $val['column_3'], '</td>';


     if (!empty($val['column_4'])){
        foreach((array) $val['column_4'] as $key){
            echo '  <td>', $key , '</td>';
        }
     }
    echo '</tr>';
}
echo '</table>';

I guess at some point $val['column_4'] is empty or not an array at all.