Difference Between Running PHP Using Fast CGI Or As An Apache Module?

I have been a web developer using PHP for years now, and my work laptop runs CentOS with PHP5 as an Apache Module. This suits me just fine for development, but now I want to setup a web server that runs PHP 4.4.8 and PHP 5.2.5 and offer hosting to people. I know PHP4 is very old, but I am looking to offer my users the freedom of choice, as there are still third party applications out there that may need to be run on PHP4.

I will want PHP 5 to be the default, and allow PHP 4 to be used if either the file extension is .php4, or they have a line in their .htaccess file. I have seen various tutorials in search engine results saying to run both, I can either do one of the following:

1. Install PHP5 as a module, and run PHP4 using FastCGI
2. Install PHP5 and PHP4 and run both using FastCGI
3. Install PHP4 as a module, and run PHP5 using FastCGI

In the future I will also be looking to support PHP6 once a stable version has been released, though that will probably be optional to begin with and require a line in the .htaccess file too - like PHP4. I will be using Apache 2.2.8 on CentOS 5.1. I am also looking to install Ruby on Rails and Django too, which I think use FastCGI.

What's the difference between running as an Apache Module, or using FastCGI? This will be for a shared hosting environment so performance over lots of connections, stability and security are my concerns. Should I run everything using FastCGI, if not, would the default PHP version be better off installed as an Apache Module?