chunk_arrays.php

<?php

$numbers = array(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20);
$grid = array_chunk($numbers, 4);

$output = '';
foreach($grid as $row)
{
 $output[] = '<tr>';  //or ul
 foreach($row as $item)
 {
  $output[] = '<td>'.$item.'</td>';
 }
 $output[] = '</tr>'; //or ul
}

echo implode($output, '');