Try this, I hope I understood your question, and you don't mean you want to draw these links from a text file:
<?php
srand();
$randNum = rand(0, 3);
if($randNum == 0) {
echo "<a href=\"http://mysite.com/url1.php\">Link1</a>";
} elseif($randNum == 1) {
echo "<a href=\"http://mysite.com/url2.php\">Link2</a>";
} elseif($randNum == 2) {
echo "<a href=\"http://mysite.com/url3.php\">Link3</a>";
} else {
echo "<a href=\"http://mysite.com/url4.php\">Link4</a>";
}
?>
Brad
*edit - this is untested, but I think it is written out right I'm fairly new to PHP 😃