The GNU General Public Licence is a software licence maintained by the Free Software Foundation. All of my scripts are free software. So the answer is yes, you can edit my scripts. You can even redistribute my scripts, edited or no. You can even sell my scripts (not sure why you'd want to).

The only caveats are that you must continue to release it under the GNU GPL and give me credit for my contributions.

VSNS Lemon 4 requires PHP 5 because it has richer support for object-oriented programming, which is part of what makes VSNS Lemon 4 such an improvement. Limiting myself to PHP 4's OOP support would have severely hampered its development. Plus, the PHP development team has announced that it will no longer support PHP 4 after the end of 2007. This means that any future projects might as well get used to using PHP 5, and web hosts should start upgrading to PHP 5 if they haven't done so already.