Assuming your server supports PHP... this is a simple way to do it:
PHP:
<?php
$pages = "
index1.html
index2.html
index3.html
index4.html
";
$indexes = explode("\n", $pages);
foreach ($indexes as $key => $line) {
$line = trim(str_replace("\r", '', $line));
if (!empty($line)) $files[] = $line;
}
$randomKey = rand(0, (count($files)-1));
include ($files[$randomKey]);
?>
(I included extra code to make it easier on you if you don't know PHP)
Just put each index file between the quotes, each on a new line, and it will include a random one of those.