foreach(array_chunk($arr1,4) as $v){
      echo "<tr>\n";
    foreach($v as $t) echo "<td>".$t."</td>\n";
    echo "</tr>\n";
};

------解决方案--------------------
$arr1=array("a","b","c","d","e","f","g"); 

$i = 1;
echo '<tr>';
foreach ($arr1 as $key=>$val)
{
	echo '<td>' , $val, '</td>';
	if($i%4 == 0) echo "</tr><tr>";
	++$i;
}
echo '</tr>';