Show recent post at static page

i add this code at functions.php

function my_recent_posts_shortcode($atts){
 $q = new WP_Query(
   array( 'orderby' => 'date', 'posts_per_page' => '4')
 );

$list = '<table style="width:100%">
  <tr>';

while($q->have_posts()) : $q->the_post();

 $list .= '<td align="center" width="25%">' . get_the_post_thumbnail($post_id, array( 108, 121)) . '<br />' . '<a href="' . get_permalink() . '">' . get_the_title() . '</a>' . '<br />' . '</td>';

endwhile;

wp_reset_query();

return $list . '</tr>
</table>';

}

add_shortcode('recent-posts', 'my_recent_posts_shortcode');

it works, but how to show 12 post with each cell is 4 post show?
pls help ??