Sorry, Windows practices usually don't work on *NIX, especially not in this case. PHP is distributed in source form.
Obviously the easiest option would be to stick with older XAMPP version (1.6.8a) and switch it to PHP 4 using lampp php4
For downgrading PHP in the latest XAMPP, I see basically two options.
You can try to move PHP files from the older XAMPP, which - my guess - won't work due to different versions of libraries (unless you move those too) and configuration files.
Another tricky option is a compilation of PHP 4 against the latest XAMPP; the process is loosely similar to the PHP 5 compilation which I have described
. You will probably have to use different configuration options, although I am not sure which (php-config-4.4.9 doesn't contain them). Also pay attention to cross-compile options in the HOWTO if you are compiling on 32-bit system.