A whole lot of nothing. Kinda. Really, there are bugs in everything, and 4.2 will have some that later versions don't. As for functionality, not a lot of difference, most likely. Most development is occuring in PHP 6 at this time.
Encourage your provider to upgrade, but gently. Truth to be told, what you write at first might have worked on PHP3; you never can tell. Books tend to lag behind; most of what you'll read would work, even in PHP5 (although 6 maybe a different story).
In particular, watch out for "register globals" (which should be OFF). However, IIRC, both the Ullman text and the SAM's book assume it is on (at least the versions I have ... YMMV; I know that the books do get upgraded from time to time. Example:
// form.html
<html>
<head>
<title>Submit your name</title>
</head>
<body>
<form method="post" action="handler.php">
Input your name:<input type="text" name="myname">
<input type="submit" value="Submit">
</form>
</body>
</html>
// handler.php
<?php
echo "Hello, $name"; // this will not work if "register_globals" is OFF
echo "Hello, ".$_POST['name']; // this will
?>
Oh, and BTW: Welcome to PHPBuilder!
Also, show us your Strat ;-)
