Ciao, prova con questo codice
$query_1=mysql_query("SELECT * FROM prodotti")
or die("errore nella query;".mysql_error());
echo"<table border='1' cellspacing='0' cellpadding='0'>";
while($num=mysql_fetch_array($query_1))
{
if($num['quantitativo'] < 5)
{
echo"<tr>
<td>
<font size='1' face='Verdana'>
$num[nome]
</font>
</td>
<td>
<font size='1' face='Verdana'>
$num[prezzo]
</font>
</td>
<td>
<font size='1' face='Verdana' color='#DF2020'>
$num[quantitativo]
</font>
</td>
<td>
<font size='1' face='Verdana'>
$num[codice]
</font>
</td>
</tr>";
}
}
echo"</table>";