PHP4 end of life was at the end of 2007. I don't think you'd lose many users for requiring php5. (sorry couldn't reply to the correct nested comment)

That's a good argument, I thought it was still a supported version. Thank you, I will amend the article.

For maximum use I think PHP4 is the sweet spot, it's more than enough to build a relatively simple project as this and it would help to not require people to upgrade to the latest and greatest version of PHP if they happen to have a slightly older installation.

Personally I can't stand it when for a simple module I have to go and make changes to a system that is already working, and I can imagine that people would want to take this and integrate it as a module in their own website.

If there are really urgent reasons why PHP5 would be the only acceptable platform for something like this then I'm open to that but current+previous major release seem to be a good place to be in terms of installed base.