You'll still be able to use PHP4 for your scripts, but there isn't a button to switch back to PHP4. It can be done using the account .htaccess file:
AddHandler application/x-httpd-php4 .php
Any extensions needing to be parsed as PHP4 should be listed there.
Alternatively, you can use this code to hard code any set extension to PHP5:
AddHandler application/x-httpd-php5 .php
It's the default for PHP on the servers switching over, but in case you had any set file extensions needing PHP5 versus those needing PHP4, I wanted to give the code for both PHP4 and PHP5 reference.
As for PHP 5.3.1 and 5.3.2, there's no word on when it's going to be the default, since it does indeed produce errors for some sites. There is a patch in cPanel's EasyApache to silence those deprecated warnings, though, under the PHP5 Exhaustive Options list:
Silence Deprecated Patch (This patch will remove E_DEPRECATED and E_USER_DEPRECATED from E_ALL)